Refactor: share nav drag and drop logic

Backend/internal/bootstrap/service.go

@@ -841,19 +841,34 @@ func (service *Service) loadProjectByID(ctx context.Context, projectID string) (
 }
 
 func (service *Service) GetProjectHierarchyFolders(ctx context.Context, projectID string) ([]ProjectHierarchyFolderRecord, error) {
+	return service.getProjectHierarchyFoldersByRootPath(ctx, projectID, projectHierarchyRootPath)
+}
+
+func (service *Service) GetProjectTreeFolders(ctx context.Context, projectID string) ([]ProjectHierarchyFolderRecord, error) {
+	return service.getProjectHierarchyFoldersByRootPath(ctx, projectID, projectTreeRootPath)
+}
+
+func (service *Service) getProjectHierarchyFoldersByRootPath(
+	ctx context.Context,
+	projectID string,
+	rootPath func(projectSlug string) string,
+) ([]ProjectHierarchyFolderRecord, error) {
 	project, err := service.loadProjectByID(ctx, projectID)
 	if err != nil {
 		return nil, err
 	}
 
+	rootParentPath := rootPath(project.Slug)
+
 	rows, err := service.db.Pool.Query(ctx, `
 		SELECT path, COALESCE(parent_path, ''), COALESCE(resource_name, '')
 		FROM posix_nodes
 		WHERE node_kind = 'directory'::posix_node_kind
 		  AND logical_type = 'hierarchy_folder'
 		  AND project_slug = $1
+		  AND path LIKE $2
 		ORDER BY depth ASC, path ASC;
-	`, project.Slug)
+	`, project.Slug, rootParentPath+"/%")
 	if err != nil {
 		return nil, err
 	}
@@ -872,16 +887,29 @@ func (service *Service) GetProjectHierarchyFolders(ctx context.Context, projectI
 		return nil, err
 	}
 
-	return buildProjectHierarchyFolderTree(folderRows, projectHierarchyRootPath(project.Slug)), nil
+	return buildProjectHierarchyFolderTree(folderRows, rootParentPath), nil
 }
 
 func (service *Service) CreateProjectFolder(ctx context.Context, input CreateProjectFolderInput) (CreateProjectFolderResult, error) {
+	return service.createProjectHierarchyFolder(ctx, input, projectHierarchyRootPath, service.createProjectHierarchyFolderOnDisk)
+}
+
+func (service *Service) CreateProjectTreeFolder(ctx context.Context, input CreateProjectFolderInput) (CreateProjectFolderResult, error) {
+	return service.createProjectHierarchyFolder(ctx, input, projectTreeRootPath, service.createProjectTreeFolderOnDisk)
+}
+
+func (service *Service) createProjectHierarchyFolder(
+	ctx context.Context,
+	input CreateProjectFolderInput,
+	rootPath func(projectSlug string) string,
+	createOnDisk func(projectSlug, parentFolderID, name string) (string, string, error),
+) (CreateProjectFolderResult, error) {
 	project, err := service.loadProjectByID(ctx, strings.TrimSpace(input.ProjectID))
 	if err != nil {
 		return CreateProjectFolderResult{}, err
 	}
 
-	createdPath, _, err := service.createProjectHierarchyFolderOnDisk(project.Slug, strings.TrimSpace(input.ParentFolderID), input.Name)
+	createdPath, _, err := createOnDisk(project.Slug, strings.TrimSpace(input.ParentFolderID), input.Name)
 	if err != nil {
 		return CreateProjectFolderResult{}, err
 	}
@@ -890,7 +918,7 @@ func (service *Service) CreateProjectFolder(ctx context.Context, input CreatePro
 		return CreateProjectFolderResult{}, fmt.Errorf("rebuild POSIX projection: %w", err)
 	}
 
-	folders, err := service.GetProjectHierarchyFolders(ctx, project.ID)
+	folders, err := service.getProjectHierarchyFoldersByRootPath(ctx, project.ID, rootPath)
 	if err != nil {
 		return CreateProjectFolderResult{}, err
 	}
@@ -1183,8 +1211,19 @@ func (service *Service) ensureBootstrapPOSIXSkeleton(
 }
 
 func (service *Service) createProjectHierarchyFolderOnDisk(projectSlug, parentFolderID, name string) (string, string, error) {
-	rootPath := strings.TrimSpace(service.posixRoot)
-	if rootPath == "" {
+	return service.createProjectFolderOnDisk(projectSlug, parentFolderID, name, projectHierarchyRootPath)
+}
+
+func (service *Service) createProjectTreeFolderOnDisk(projectSlug, parentFolderID, name string) (string, string, error) {
+	return service.createProjectFolderOnDisk(projectSlug, parentFolderID, name, projectTreeRootPath)
+}
+
+func (service *Service) createProjectFolderOnDisk(
+	projectSlug, parentFolderID, name string,
+	rootPathBuilder func(projectSlug string) string,
+) (string, string, error) {
+	posixRoot := strings.TrimSpace(service.posixRoot)
+	if posixRoot == "" {
 		return "", "", fmt.Errorf("POSIX root is not configured")
 	}
 
@@ -1193,14 +1232,12 @@ func (service *Service) createProjectHierarchyFolderOnDisk(projectSlug, parentFo
 		return "", "", fmt.Errorf("folder name is required")
 	}
 
-	projectRoot := filepath.Join(rootPath, "projects", slugDir("project", projectSlug))
-	childrenRoot := filepath.Join(projectRoot, "children")
-	parentDir := childrenRoot
-	containerProjectionPath := projectHierarchyRootPath(projectSlug)
+	containerProjectionPath := rootPathBuilder(projectSlug)
+	parentDir := filepath.Join(posixRoot, filepath.FromSlash(containerProjectionPath))
 
 	if strings.TrimSpace(parentFolderID) != "" {
 		containerProjectionPath = filepath.ToSlash(filepath.Join(strings.TrimSpace(parentFolderID), "children"))
-		parentDir = filepath.Join(rootPath, filepath.FromSlash(containerProjectionPath))
+		parentDir = filepath.Join(posixRoot, filepath.FromSlash(containerProjectionPath))
 		info, err := os.Stat(parentDir)
 		if err != nil {
 			if os.IsNotExist(err) {
@@ -1324,6 +1361,10 @@ func projectHierarchyRootPath(projectSlug string) string {
 	return filepath.ToSlash(filepath.Join("projects", slugDir("project", projectSlug), "children"))
 }
 
+func projectTreeRootPath(projectSlug string) string {
+	return filepath.ToSlash(filepath.Join("projects", slugDir("project", projectSlug), "tree"))
+}
+
 func normalizePOSIXSlug(value string) string {
 	trimmed := strings.TrimSpace(strings.ToLower(value))
 	if trimmed == "" {

Backend/internal/bootstrap/service_test.go

@@ -161,6 +161,56 @@ func TestCreateProjectHierarchyFolderOnDiskCreatesExpectedFolderShape(t *testing
 	}
 }
 
+func TestCreateProjectTreeFolderOnDiskCreatesExpectedFolderShape(t *testing.T) {
+	rootPath := filepath.Join(t.TempDir(), "POSIX")
+	service := NewService(nil, rootPath)
+
+	err := service.ensureBootstrapPOSIXSkeleton(
+		InstallationRecord{ID: "installation-1", Name: "MangoPig", Mode: "personal", Access: "local", Protocol: "http", Host: "localhost", IsBootstrapped: true},
+		AdminSummary{ID: "admin-1", Email: "ronald@example.com", DisplayName: "Ronald"},
+		namedRecord{ID: "org-1", Name: "Primary Organization", Slug: "primary-organization"},
+		namedRecord{ID: "dept-1", Name: "Primary Department", Slug: "primary-department"},
+		namedRecord{ID: "team-1", Name: "Primary Team", Slug: "primary-team"},
+		namedRecord{ID: "project-1", Name: "Primary Project", Slug: "primary-project"},
+	)
+	if err != nil {
+		t.Fatalf("ensure bootstrap POSIX skeleton: %v", err)
+	}
+
+	createdPath, createdSlug, err := service.createProjectTreeFolderOnDisk("primary-project", "", "Docs")
+	if err != nil {
+		t.Fatalf("createProjectTreeFolderOnDisk root folder: %v", err)
+	}
+	if createdPath != "projects/project-primary-project/tree/folder-docs" {
+		t.Fatalf("unexpected created path: %s", createdPath)
+	}
+	if createdSlug != "docs" {
+		t.Fatalf("unexpected created slug: %s", createdSlug)
+	}
+
+	createdFolderPath := filepath.Join(rootPath, "projects", "project-primary-project", "tree", "folder-docs")
+	for _, path := range []string{
+		filepath.Join(createdFolderPath, "folder.json"),
+		filepath.Join(createdFolderPath, "acl.json"),
+		filepath.Join(createdFolderPath, "children"),
+	} {
+		if _, err := os.Stat(path); err != nil {
+			t.Fatalf("expected path to exist %s: %v", path, err)
+		}
+	}
+
+	nestedPath, nestedSlug, err := service.createProjectTreeFolderOnDisk("primary-project", createdPath, "Research")
+	if err != nil {
+		t.Fatalf("createProjectTreeFolderOnDisk nested folder: %v", err)
+	}
+	if nestedPath != "projects/project-primary-project/tree/folder-docs/children/folder-research" {
+		t.Fatalf("unexpected nested path: %s", nestedPath)
+	}
+	if nestedSlug != "research" {
+		t.Fatalf("unexpected nested slug: %s", nestedSlug)
+	}
+}
+
 func TestBuildProjectHierarchyFolderTreeBuildsNestedStructure(t *testing.T) {
 	rows := []projectHierarchyFolderRow{
 		{Path: "projects/project-primary-project/children/folder-design", ParentPath: projectHierarchyRootPath("primary-project"), Label: "Design"},

Backend/internal/httpx/api_project_routes.go

@@ -79,6 +79,68 @@ func (routes apiRoutes) handleCreateProjectFolder(w http.ResponseWriter, r *http
 	})
 }
 
+func (routes apiRoutes) handleProjectTreeFolders(w http.ResponseWriter, r *http.Request) {
+	projectID := strings.TrimSpace(chi.URLParam(r, "projectId"))
+	if projectID == "" {
+		WriteError(w, http.StatusBadRequest, RequestIDFromContext(r.Context()), "invalid_request", "Project ID is required.")
+		return
+	}
+
+	folders, err := routes.bootstrapService().GetProjectTreeFolders(r.Context(), projectID)
+	if err != nil {
+		routes.writeProjectFolderError(w, r, err)
+		return
+	}
+
+	WriteJSON(w, http.StatusOK, map[string]any{
+		"data": map[string]any{
+			"projectId": projectID,
+			"folders":   folders,
+		},
+		"meta": map[string]any{
+			"resource": "project-tree-folders",
+		},
+	})
+}
+
+func (routes apiRoutes) handleCreateProjectTreeFolder(w http.ResponseWriter, r *http.Request) {
+	projectID := strings.TrimSpace(chi.URLParam(r, "projectId"))
+	if projectID == "" {
+		WriteError(w, http.StatusBadRequest, RequestIDFromContext(r.Context()), "invalid_request", "Project ID is required.")
+		return
+	}
+
+	payload, ok := decodeProjectFolderRequest(w, r)
+	if !ok {
+		return
+	}
+
+	payload.Name = strings.TrimSpace(payload.Name)
+	payload.ParentFolderID = strings.TrimSpace(payload.ParentFolderID)
+	if payload.Name == "" {
+		WriteError(w, http.StatusBadRequest, RequestIDFromContext(r.Context()), "invalid_request", "Folder name is required.")
+		return
+	}
+
+	result, err := routes.bootstrapService().CreateProjectTreeFolder(r.Context(), bootstrapservice.CreateProjectFolderInput{
+		ProjectID:      projectID,
+		ParentFolderID: payload.ParentFolderID,
+		Name:           payload.Name,
+	})
+	if err != nil {
+		routes.writeProjectFolderError(w, r, err)
+		return
+	}
+
+	WriteJSON(w, http.StatusCreated, map[string]any{
+		"data": result,
+		"meta": map[string]any{
+			"resource":  "project-tree-folder-create",
+			"persisted": true,
+		},
+	})
+}
+
 func (routes apiRoutes) writeProjectFolderError(w http.ResponseWriter, r *http.Request, err error) {
 	switch {
 	case errors.Is(err, bootstrapservice.ErrProjectNotFound), errors.Is(err, bootstrapservice.ErrProjectFolderNotFound):

Backend/internal/httpx/api_routes.go

@@ -36,6 +36,8 @@ func (routes apiRoutes) Register(router chi.Router) {
 		apiRouter.Route("/projects/{projectId}", func(projectRouter chi.Router) {
 			projectRouter.Get("/folders", routes.handleProjectFolders)
 			projectRouter.Post("/folders", routes.handleCreateProjectFolder)
+			projectRouter.Get("/tree/folders", routes.handleProjectTreeFolders)
+			projectRouter.Post("/tree/folders", routes.handleCreateProjectTreeFolder)
 		})
 
 		if routes.cfg.Config.IsDevelopment() {

Backend/internal/posixproj/projector.go

@@ -411,7 +411,7 @@ func classifyPath(relPath string, isDir bool) (logicalType, fileRole string) {
 				return "hierarchy_folder", ""
 			}
 			if hasTreeAncestor && strings.HasPrefix(name, "folder-") {
-				return "folder", ""
+				return "hierarchy_folder", ""
 			}
 			if hasTreeAncestor && strings.HasPrefix(name, "item-") {
 				return "item", ""
@@ -425,7 +425,7 @@ func classifyPath(relPath string, isDir bool) (logicalType, fileRole string) {
 				return "item", fileRole
 			}
 			if strings.HasPrefix(parentName, "folder-") {
-				return "folder", fileRole
+				return "hierarchy_folder", fileRole
 			}
 		}
 		return "project", fileRole

Backend/internal/posixproj/projector_test.go

@@ -16,6 +16,7 @@ func TestScanRootBuildsProjectedNodesFromBootstrapShape(t *testing.T) {
 	mustMkdirAll(t, filepath.Join(root, "projects", "project-primary-project", "children", "folder-design", "children", "project-web", "children"))
 	mustMkdirAll(t, filepath.Join(root, "projects", "project-primary-project", "children", "folder-design", "children", "project-web", "tree"))
 	mustMkdirAll(t, filepath.Join(root, "projects", "project-primary-project", "tree"))
+	mustMkdirAll(t, filepath.Join(root, "projects", "project-primary-project", "tree", "folder-docs", "children"))
 	mustMkdirAll(t, filepath.Join(root, "projects", "project-primary-project", "tree", "folder-docs", "item-roadmap"))
 	mustMkdirAll(t, filepath.Join(root, "users", "personals"))
 
@@ -185,10 +186,15 @@ func TestScanRootBuildsProjectedNodesFromBootstrapShape(t *testing.T) {
 	}
 
 	treeFolder := index["projects/project-primary-project/tree/folder-docs"]
-	if treeFolder.LogicalType != "folder" || treeFolder.ProjectSlug != "primary-project" {
+	if treeFolder.LogicalType != "hierarchy_folder" || treeFolder.ProjectSlug != "primary-project" {
 		t.Fatalf("unexpected tree folder node: %#v", treeFolder)
 	}
 
+	treeFolderACL := index["projects/project-primary-project/tree/folder-docs/folder.json"]
+	if treeFolderACL.LogicalType != "hierarchy_folder" || treeFolderACL.FileRole != "folder" {
+		t.Fatalf("unexpected tree folder file classification: %#v", treeFolderACL)
+	}
+
 	treeItem := index["projects/project-primary-project/tree/folder-docs/item-roadmap/item.json"]
 	if treeItem.LogicalType != "item" || treeItem.FileRole != "item" {
 		t.Fatalf("unexpected tree item classification: %#v", treeItem)

Frontend/src/components/shell/ProjectSelector/ProjectSelector.module.scss

@@ -1,3 +1,5 @@
+@use "../shared/tree-nav" as treeNav;
+
 .root {
   display: grid;
   --project-drawer-gap: var(--space-3);
@@ -191,12 +193,9 @@
 }
 
 .treeSectionLabel {
-  @include text-caption;
+	@include treeNav.section-label;
   margin: 0;
   padding: 0 var(--space-3);
-  color: var(--color-text-subtle);
-  text-transform: uppercase;
-  letter-spacing: 0.04em;
 }
 
 .treeSectionHeader {
@@ -248,133 +247,72 @@
 }
 
 .treeList {
-  list-style: none;
-  display: grid;
-  gap: var(--space-1);
-  padding: 0;
+	@include treeNav.tree-list;
 }
 
 .treeEmptySlot {
-	min-height: calc(var(--control-size-lg) - var(--space-2));
-	padding-left: calc(var(--space-3) + (var(--tree-depth, 0) * var(--space-4)));
-	border-radius: var(--radius-lg);
-	border: 1px dashed color-mix(in srgb, var(--color-border) 38%, transparent);
-	opacity: 0.35;
+	@include treeNav.empty-slot;
 }
 
 .treeInputRow {
-	width: 100%;
-	min-width: 0;
-	display: grid;
-	grid-template-columns: auto minmax(0, 1fr);
-	align-items: center;
-	gap: var(--space-2);
-	min-height: calc(var(--control-size-lg) - var(--space-2));
-	padding: var(--space-2) var(--space-3);
-	padding-left: calc(var(--space-3) + (var(--tree-depth, 0) * var(--space-4)));
-	border: 1px solid color-mix(in srgb, var(--color-border) 42%, transparent);
-	border-radius: var(--radius-lg);
-	background: color-mix(in srgb, var(--color-surface) 94%, transparent);
+	@include treeNav.input-row;
 }
 
 .treeInput {
-	width: 100%;
-	min-width: 0;
-	border: 0;
-	background: transparent;
-	color: var(--color-text);
-	font: inherit;
-	outline: none;
-}
-
-.treeInput::placeholder {
-	color: var(--color-text-muted);
+	@include treeNav.input;
 }
 
 .treeItem {
-  width: 100%;
-  min-width: 0;
-  display: grid;
-  grid-template-columns: auto auto minmax(0, 1fr) auto;
-  align-items: center;
-  gap: var(--space-2);
-  min-height: calc(var(--control-size-lg) - var(--space-2));
-  padding: var(--space-2) var(--space-3);
-  padding-left: calc(var(--space-3) + (var(--tree-depth, 0) * var(--space-4)));
-  border: 1px solid transparent;
-  border-radius: var(--radius-lg);
-  background: transparent;
-  color: var(--color-text-muted);
-  transition:
-    background 160ms var(--easing-standard),
-    color 160ms var(--easing-standard),
-    border-color 160ms var(--easing-standard),
-    box-shadow 160ms var(--easing-standard),
-    transform 180ms var(--easing-standard);
-  text-align: left;
+	@include treeNav.item;
 }
 
 .treeItem:hover,
 .treeItem:focus-visible {
-  background: color-mix(in srgb, var(--color-surface-hover) 80%, var(--color-accent-soft) 20%);
-  color: var(--color-text);
-  box-shadow: inset 0 1px 0 color-mix(in srgb, white 4%, transparent);
+	@include treeNav.item-hover;
 }
 
 .treeItemFolder {
-  color: var(--color-text);
+	@include treeNav.item-folder;
 }
 
 .treeItemDragging {
-	opacity: 0.45;
-	transform: scale(0.985);
-	box-shadow: none;
+	@include treeNav.item-dragging;
 }
 
 .treeItemDropBefore {
-	box-shadow: inset 0 2px 0 color-mix(in srgb, var(--color-accent-strong) 78%, transparent);
+	@include treeNav.item-drop-before;
 }
 
 .treeItemDropAfter {
-	box-shadow: inset 0 -2px 0 color-mix(in srgb, var(--color-accent-strong) 78%, transparent);
+	@include treeNav.item-drop-after;
 }
 
 .treeItemDropInside {
-	border-color: color-mix(in srgb, var(--color-accent-strong) 55%, transparent);
-	background: color-mix(in srgb, var(--color-accent-soft) 36%, var(--color-surface));
-	color: var(--color-text);
-	box-shadow: inset 0 1px 0 color-mix(in srgb, white 4%, transparent);
+	@include treeNav.item-drop-inside;
 }
 
 .folderChevron {
-  color: var(--color-text-muted);
-  transition: transform 160ms var(--easing-standard);
+	@include treeNav.folder-chevron;
 }
 
 .folderChevronOpen {
-  transform: rotate(90deg);
+	@include treeNav.folder-chevron-open;
 }
 
 .treeItemActive {
-  border-color: var(--color-border);
-  background: var(--color-surface);
-  color: var(--color-text);
-  box-shadow: inset 0 1px 0 color-mix(in srgb, white 4%, transparent);
+	@include treeNav.item-active;
 }
 
 .icon {
-  color: inherit;
-  opacity: 0.85;
+	@include treeNav.icon;
 }
 
 .label {
-  @include text-label;
-  min-width: 0;
+	@include treeNav.label;
 }
 
 .itemMeta {
-  @include text-caption;
-  color: var(--color-text-muted);
+	@include treeNav.item-meta;
 }
 
 @media (max-width: 720px) {

Frontend/src/components/shell/ProjectSelector/ProjectSelector.tsx

@@ -5,6 +5,17 @@ import { ChevronDown, ChevronRight, Folder, LayoutGrid, ListCollapse, UnfoldVert
 import { ProjectContextMenu } from "../ProjectContextMenu/ProjectContextMenu";
 import { useAppShellData } from "../data/app-shell.context";
 import { resolveAPIBase } from "../../../lib/api";
+import {
+	collectBranchNodeIds,
+	findTreeNodeDepth,
+	findTreeNodeLocation,
+	getPointerRelativeY,
+	isUuidString,
+	moveTreeNode,
+	resolveTreeDropTarget,
+	type NavTreeAdapter,
+	type NavTreeDropTarget,
+} from "../shared/navTreeDnd";
 import {
 	createProjectFolderTarget,
 	createProjectSurfaceTarget,
@@ -56,24 +67,13 @@ type PendingProjectFolderDraft = {
 	depth: number;
 };
 
-type ProjectDragTarget = {
-	parentId: string | null;
-	index: number;
-	intent: "before" | "after" | "inside";
-	targetNodeId?: string;
-};
+type ProjectDragTarget = NavTreeDropTarget;
 
 type ProjectDragState = {
 	draggedNodeId: string;
 	dropTarget: ProjectDragTarget | null;
 };
 
-type ProjectNodeLocation = {
-	parentId: string | null;
-	index: number;
-	node: ProjectTreeNode;
-};
-
 const LONG_PRESS_MS = 320;
 
 const getProjectTreeNodeId = (node: ProjectTreeNode): string =>
@@ -98,198 +98,20 @@ const buildProjectTree = (
 	...buildPersistedFolderNodes(folders),
 ];
 
-const collectProjectFolderIds = (nodes: readonly ProjectTreeNode[]): string[] => {
-	const ids: string[] = [];
-
-	for (const node of nodes) {
-		if (node.kind !== "folder") {
-			continue;
-		}
-
-		ids.push(node.id);
-		ids.push(...collectProjectFolderIds(node.children));
-	}
-
-	return ids;
-};
-
 const readPersistedFolders = (body: ProjectFoldersResponse): PersistedProjectFolderRecord[] =>
 	Array.isArray(body.data?.folders) ? body.data.folders : [];
 
-const cloneProjectTreeNode = (node: ProjectTreeNode): ProjectTreeNode => {
-	if (node.kind === "project") {
-		return {
-			kind: "project",
-			item: { ...node.item },
-		};
-	}
-
-	return {
-		kind: "folder",
-		id: node.id,
-		label: node.label,
-		meta: node.meta,
-		children: node.children.map(cloneProjectTreeNode),
-	};
-};
-
-const findProjectNodeLocation = (
-	nodes: readonly ProjectTreeNode[],
-	nodeId: string,
-	parentId: string | null = null,
-): ProjectNodeLocation | null => {
-	for (let index = 0; index < nodes.length; index += 1) {
-		const node = nodes[index];
-
-		if (getProjectTreeNodeId(node) === nodeId) {
-			return { parentId, index, node };
-		}
-
-		if (node.kind === "folder") {
-			const nestedLocation = findProjectNodeLocation(node.children, nodeId, node.id);
-
-			if (nestedLocation) {
-				return nestedLocation;
-			}
-		}
-	}
-
-	return null;
-};
-
-const findProjectNodeDepth = (nodes: readonly ProjectTreeNode[], nodeId: string, depth = 0): number | null => {
-	for (const node of nodes) {
-		if (getProjectTreeNodeId(node) === nodeId) {
-			return depth;
-		}
-
-		if (node.kind === "folder") {
-			const nestedDepth = findProjectNodeDepth(node.children, nodeId, depth + 1);
-
-			if (nestedDepth !== null) {
-				return nestedDepth;
-			}
-		}
-	}
-
-	return null;
-};
-
-const projectTreeContainsNode = (nodes: readonly ProjectTreeNode[], nodeId: string): boolean => {
-	for (const node of nodes) {
-		if (getProjectTreeNodeId(node) === nodeId) {
-			return true;
-		}
-
-		if (node.kind === "folder" && projectTreeContainsNode(node.children, nodeId)) {
-			return true;
-		}
-	}
-
-	return false;
-};
-
-const removeProjectTreeNode = (
-	nodes: readonly ProjectTreeNode[],
-	nodeId: string,
-): { nodes: ProjectTreeNode[]; removed: ProjectTreeNode | null } => {
-	const nextNodes: ProjectTreeNode[] = [];
-	let removed: ProjectTreeNode | null = null;
-
-	for (const node of nodes) {
-		if (getProjectTreeNodeId(node) === nodeId) {
-			removed = node;
-			continue;
-		}
-
-		if (node.kind === "folder") {
-			const result = removeProjectTreeNode(node.children, nodeId);
-
-			if (result.removed) {
-				removed = result.removed;
-				nextNodes.push({
-					...node,
-					children: result.nodes,
-				});
-				continue;
-			}
-		}
-
-		nextNodes.push(node);
-	}
-
-	return { nodes: nextNodes, removed };
-};
-
-const insertProjectTreeNode = (
-	nodes: readonly ProjectTreeNode[],
-	parentId: string | null,
-	index: number,
-	nodeToInsert: ProjectTreeNode,
-): ProjectTreeNode[] => {
-	if (parentId === null) {
-		const nextNodes = [...nodes];
-		nextNodes.splice(Math.max(0, Math.min(index, nextNodes.length)), 0, nodeToInsert);
-		return nextNodes;
-	}
-
-	return nodes.map((node) => {
-		if (node.kind !== "folder") {
-			return node;
-		}
-
-		if (node.id === parentId) {
-			const nextChildren = [...node.children];
-			nextChildren.splice(Math.max(0, Math.min(index, nextChildren.length)), 0, nodeToInsert);
-			return {
+const projectTreeAdapter: NavTreeAdapter<ProjectTreeNode> = {
+	getNodeId: getProjectTreeNodeId,
+	isBranchNode: (node) => node.kind === "folder",
+	getChildren: (node) => (node.kind === "folder" ? node.children : []),
+	withChildren: (node, children) =>
+		node.kind === "folder"
+			? {
 				...node,
-				children: nextChildren,
-			};
-		}
-
-		return {
-			...node,
-			children: insertProjectTreeNode(node.children, parentId, index, nodeToInsert),
-		};
-	});
-};
-
-const moveProjectTreeNode = (
-	nodes: readonly ProjectTreeNode[],
-	draggedNodeId: string,
-	dropTarget: ProjectDragTarget,
-): ProjectTreeNode[] => {
-	const location = findProjectNodeLocation(nodes, draggedNodeId);
-
-	if (!location) {
-		return [...nodes];
-	}
-
-	if (
-		location.node.kind === "folder" &&
-		dropTarget.parentId !== null &&
-		(projectTreeContainsNode(location.node.children, dropTarget.parentId) || dropTarget.parentId === location.node.id)
-	) {
-		return [...nodes];
-	}
-
-	let normalizedIndex = dropTarget.index;
-
-	if (dropTarget.parentId === location.parentId && dropTarget.index > location.index) {
-		normalizedIndex -= 1;
-	}
-
-	if (dropTarget.parentId === location.parentId && normalizedIndex === location.index) {
-		return [...nodes];
-	}
-
-	const removalResult = removeProjectTreeNode(nodes, draggedNodeId);
-
-	if (!removalResult.removed) {
-		return [...nodes];
-	}
-
-	return insertProjectTreeNode(removalResult.nodes, dropTarget.parentId, normalizedIndex, removalResult.removed);
+				children: [...children],
+			}
+			: node,
 };
 
 const ProjectFolderDraftRow = (props: {
@@ -385,18 +207,18 @@ const ProjectFolderBranch = (props: {
 									[styles.treeItemFolder]: true,
 									[styles.treeItemDragging]: isDraggedNode(),
 									[styles.treeItemDropBefore]: dropIntent() === "before",
-								[styles.treeItemDropAfter]: dropIntent() === "after",
-								[styles.treeItemDropInside]: dropIntent() === "inside",
-							}}
-							style={{ "--tree-depth": String(props.depth) }}
-							aria-expanded={!isCollapsed()}
-							onClick={() => {
-								if (props.dragState || props.isTreeClickSuppressed()) {
-									return;
-								}
+									[styles.treeItemDropAfter]: dropIntent() === "after",
+									[styles.treeItemDropInside]: dropIntent() === "inside",
+								}}
+								style={{ "--tree-depth": String(props.depth) }}
+								aria-expanded={!isCollapsed()}
+								onClick={() => {
+									if (props.dragState || props.isTreeClickSuppressed()) {
+										return;
+									}
 
-								props.onToggleFolder(node.id);
-							}}
+									props.onToggleFolder(node.id);
+								}}
 								onContextMenu={(event): void => props.onOpenFolderMenu(event, node)}
 								onPointerDown={(event): void => props.onNodePointerDown(event, node.id)}
 								onPointerMove={(event): void =>
@@ -550,7 +372,7 @@ export const ProjectSelector = (props: ProjectSelectorProps): JSX.Element => {
 
 	const syncProjectTree = (): void => {
 		const nextTree = buildProjectTree(appShellData.projectItems(), persistedFolders());
-		const availableFolderIds = new Set(collectProjectFolderIds(nextTree));
+		const availableFolderIds = new Set(collectBranchNodeIds(nextTree, projectTreeAdapter));
 
 		setProjectTreeNodes(nextTree);
 		setCollapsedFolderIds((current) => current.filter((folderId) => availableFolderIds.has(folderId)));
@@ -563,7 +385,7 @@ export const ProjectSelector = (props: ProjectSelectorProps): JSX.Element => {
 		setDragState(null);
 	};
 
-	const folderIds = (): string[] => collectProjectFolderIds(projectTreeNodes());
+	const folderIds = (): string[] => collectBranchNodeIds(projectTreeNodes(), projectTreeAdapter);
 
 	const expandAllFolders = (): void => {
 		setCollapsedFolderIds([]);
@@ -599,6 +421,11 @@ export const ProjectSelector = (props: ProjectSelectorProps): JSX.Element => {
 			return;
 		}
 
+		if (!isUuidString(projectId)) {
+			setPersistedFolders([]);
+			return;
+		}
+
 		try {
 			const response = await fetch(`${resolveAPIBase()}/projects/${projectId}/folders`, {
 				headers: {
@@ -713,7 +540,7 @@ export const ProjectSelector = (props: ProjectSelectorProps): JSX.Element => {
 
 			suppressTreeClickTemporarily();
 			setProjectTreeNodes((current) =>
-				moveProjectTreeNode(current, nextDragState.draggedNodeId, nextDragState.dropTarget as ProjectDragTarget),
+				moveTreeNode(current, nextDragState.draggedNodeId, nextDragState.dropTarget as ProjectDragTarget, projectTreeAdapter),
 			);
 			setDragState(null);
 		};
@@ -765,7 +592,7 @@ export const ProjectSelector = (props: ProjectSelectorProps): JSX.Element => {
 	};
 
 	const selectProject = (projectId: string): void => {
-		const location = findProjectNodeLocation(projectTreeNodes(), projectId);
+		const location = findTreeNodeLocation(projectTreeNodes(), projectId, projectTreeAdapter);
 
 		if (!location || location.node.kind !== "project") {
 			return;
@@ -787,6 +614,7 @@ export const ProjectSelector = (props: ProjectSelectorProps): JSX.Element => {
 	const submitPendingFolder = async (): Promise<void> => {
 		const name = pendingFolderName().trim();
 		const draft = pendingFolderDraft();
+		const projectId = selectedProject().id;
 
 		if (!draft) {
 			return;
@@ -798,8 +626,13 @@ export const ProjectSelector = (props: ProjectSelectorProps): JSX.Element => {
 			return;
 		}
 
+		if (!isUuidString(projectId)) {
+			cancelPendingFolder();
+			return;
+		}
+
 		try {
-			const response = await fetch(`${resolveAPIBase()}/projects/${selectedProject().id}/folders`, {
+			const response = await fetch(`${resolveAPIBase()}/projects/${projectId}/folders`, {
 				method: "POST",
 				headers: {
 					Accept: "application/json",
@@ -835,16 +668,16 @@ export const ProjectSelector = (props: ProjectSelectorProps): JSX.Element => {
 			return;
 		}
 
-		switch (target.kind) {
+			switch (target.kind) {
 			case "surface":
 				beginFolderDraft(null, 0);
 				return;
 			case "folder":
-				beginFolderDraft(target.id, (findProjectNodeDepth(projectTreeNodes(), target.id) ?? 0) + 1);
+				beginFolderDraft(target.id, (findTreeNodeDepth(projectTreeNodes(), target.id, projectTreeAdapter) ?? 0) + 1);
 				return;
 			case "project": {
-				const parentId = findProjectNodeLocation(projectTreeNodes(), target.id)?.parentId ?? null;
-				beginFolderDraft(parentId, parentId ? (findProjectNodeDepth(projectTreeNodes(), parentId) ?? 0) + 1 : 0);
+				const parentId = findTreeNodeLocation(projectTreeNodes(), target.id, projectTreeAdapter)?.parentId ?? null;
+				beginFolderDraft(parentId, parentId ? (findTreeNodeDepth(projectTreeNodes(), parentId, projectTreeAdapter) ?? 0) + 1 : 0);
 				return;
 			}
 		}
@@ -886,50 +719,20 @@ export const ProjectSelector = (props: ProjectSelectorProps): JSX.Element => {
 			return;
 		}
 
-		const currentTarget = event.currentTarget;
-		if (!(currentTarget instanceof HTMLElement)) {
+		const relativeY = getPointerRelativeY(event);
+		if (relativeY === null) {
 			return;
 		}
 
-		const bounds = currentTarget.getBoundingClientRect();
-		const relativeY = bounds.height <= 0 ? 0.5 : (event.clientY - bounds.top) / bounds.height;
-		let nextTarget: ProjectDragTarget;
-
-		if (node.kind === "folder") {
-			if (relativeY < 0.28) {
-				nextTarget = {
-					parentId,
-					index,
-					intent: "before",
-					targetNodeId: node.id,
-				};
-			} else if (relativeY > 0.72) {
-				nextTarget = {
-					parentId,
-					index: index + 1,
-					intent: "after",
-					targetNodeId: node.id,
-				};
-			} else {
-				nextTarget = {
-					parentId: node.id,
-					index: node.children.length,
-					intent: "inside",
-					targetNodeId: node.id,
-				};
-			}
-		} else {
-			nextTarget = {
-				parentId,
-				index: relativeY < 0.5 ? index : index + 1,
-				intent: relativeY < 0.5 ? "before" : "after",
-				targetNodeId: node.item.id,
-			};
-		}
-
 		setDragState({
 			...nextDragState,
-			dropTarget: nextTarget,
+			dropTarget: resolveTreeDropTarget({
+				parentId,
+				index,
+				node,
+				relativeY,
+				adapter: projectTreeAdapter,
+			}),
 		});
 	};
 

Frontend/src/components/shell/WorkspaceSidebar/WorkspaceSidebar.module.scss

@@ -1,3 +1,5 @@
+@use "../shared/tree-nav" as treeNav;
+
 .sidebar {
   --sidebar-nav-item-min-height: var(--control-size-lg);
   position: relative;
@@ -117,56 +119,25 @@
 }
 
 .treeSectionLabel {
-  @include text-caption;
+	@include treeNav.section-label;
   margin: var(--space-3) 0 var(--space-2);
   padding: 0 var(--space-3);
-  color: var(--color-text-subtle);
-  text-transform: uppercase;
-  letter-spacing: 0.04em;
 }
 
 .treeList {
-  list-style: none;
-  display: grid;
-  gap: var(--space-1);
-  padding: 0;
+	@include treeNav.tree-list;
 }
 
 .treeEmptySlot {
-	min-height: calc(var(--control-size-lg) - var(--space-2));
-	padding-left: calc(var(--space-3) + (var(--tree-depth, 0) * var(--space-4)));
-	border-radius: var(--radius-lg);
-	border: 1px dashed color-mix(in srgb, var(--color-border) 38%, transparent);
-	opacity: 0.35;
+	@include treeNav.empty-slot;
 }
 
 .treeInputRow {
-	width: 100%;
-	min-width: 0;
-	display: grid;
-	grid-template-columns: auto minmax(0, 1fr);
-	align-items: center;
-	gap: var(--space-2);
-	min-height: calc(var(--control-size-lg) - var(--space-2));
-	padding: var(--space-2) var(--space-3);
-	padding-left: calc(var(--space-3) + (var(--tree-depth, 0) * var(--space-4)));
-	border: 1px solid color-mix(in srgb, var(--color-border) 42%, transparent);
-	border-radius: var(--radius-lg);
-	background: color-mix(in srgb, var(--color-surface) 94%, transparent);
+	@include treeNav.input-row;
 }
 
 .treeInput {
-	width: 100%;
-	min-width: 0;
-	border: 0;
-	background: transparent;
-	color: var(--color-text);
-	font: inherit;
-	outline: none;
-}
-
-.treeInput::placeholder {
-	color: var(--color-text-muted);
+	@include treeNav.input;
 }
 
 .navItem {
@@ -184,74 +155,44 @@
 }
 
 .treeItem {
-  width: 100%;
-  min-width: 0;
-  display: grid;
-  grid-template-columns: auto auto minmax(0, 1fr) auto;
-  align-items: center;
-  gap: var(--space-2);
-  min-height: calc(var(--control-size-lg) - var(--space-2));
-  padding: var(--space-2) var(--space-3);
-  padding-left: calc(var(--space-3) + (var(--tree-depth, 0) * var(--space-4)));
-  border: 1px solid transparent;
-  border-radius: var(--radius-lg);
-  background: transparent;
-  color: var(--color-text-muted);
-  text-align: left;
-  transition:
-    background 160ms var(--easing-standard),
-    color 160ms var(--easing-standard),
-    border-color 160ms var(--easing-standard),
-    box-shadow 160ms var(--easing-standard),
-    transform 180ms var(--easing-standard);
+	@include treeNav.item;
 }
 
 .treeItem:hover,
 .treeItem:focus-visible {
-  background: color-mix(in srgb, var(--color-surface-hover) 80%, var(--color-accent-soft) 20%);
-  color: var(--color-text);
-  box-shadow: inset 0 1px 0 color-mix(in srgb, white 4%, transparent);
+	@include treeNav.item-hover;
 }
 
 .treeItemFolder {
-  color: var(--color-text);
+	@include treeNav.item-folder;
 }
 
 .treeItemDragging {
-	opacity: 0.45;
-	transform: scale(0.985);
-	box-shadow: none;
+	@include treeNav.item-dragging;
 }
 
 .treeItemDropBefore {
-	box-shadow: inset 0 2px 0 color-mix(in srgb, var(--color-accent-strong) 78%, transparent);
+	@include treeNav.item-drop-before;
 }
 
 .treeItemDropAfter {
-	box-shadow: inset 0 -2px 0 color-mix(in srgb, var(--color-accent-strong) 78%, transparent);
+	@include treeNav.item-drop-after;
 }
 
 .treeItemDropInside {
-	border-color: color-mix(in srgb, var(--color-accent-strong) 55%, transparent);
-	background: color-mix(in srgb, var(--color-accent-soft) 36%, var(--color-surface));
-	color: var(--color-text);
-	box-shadow: inset 0 1px 0 color-mix(in srgb, white 4%, transparent);
+	@include treeNav.item-drop-inside;
 }
 
 .folderChevron {
-  color: var(--color-text-muted);
-  transition: transform 160ms var(--easing-standard);
+	@include treeNav.folder-chevron;
 }
 
 .folderChevronOpen {
-  transform: rotate(90deg);
+	@include treeNav.folder-chevron-open;
 }
 
 .treeItemActive {
-  border-color: var(--color-border);
-  background: var(--color-surface);
-  color: var(--color-text);
-  box-shadow: inset 0 1px 0 color-mix(in srgb, white 4%, transparent);
+	@include treeNav.item-active;
 }
 
 .navItemActive {
@@ -262,18 +203,15 @@
 }
 
 .icon {
-  color: inherit;
-  opacity: 0.85;
+	@include treeNav.icon;
 }
 
 .label {
-  @include text-label;
-  min-width: 0;
+	@include treeNav.label;
 }
 
 .itemMeta {
-  @include text-caption;
-  color: var(--color-text-muted);
+	@include treeNav.item-meta;
 }
 
 .sidebarCollapsed {

Frontend/src/components/shell/WorkspaceSidebar/WorkspaceSidebar.tsx

@@ -1,9 +1,20 @@
 // Path: Frontend/src/components/shell/WorkspaceSidebar/WorkspaceSidebar.tsx
 
 import { For, Show, createEffect, createSignal, onCleanup, onMount, type JSX } from "solid-js";
-import { ChevronLeft, ChevronRight, Folder } from "../../../lib/icons";
+import { resolveAPIBase } from "../../../lib/api";
+import { ChevronLeft, ChevronRight, Folder, ListCollapse, UnfoldVertical } from "../../../lib/icons";
 import { useAppShellData } from "../data/app-shell.context";
 import { ProjectSelector } from "../ProjectSelector/ProjectSelector";
+import {
+	collectBranchNodeIds,
+	findTreeNodeDepth,
+	getPointerRelativeY,
+	isUuidString,
+	moveTreeNode,
+	resolveTreeDropTarget,
+	type NavTreeAdapter,
+	type NavTreeDropTarget,
+} from "../shared/navTreeDnd";
 import {
 	createWorkspaceStaticTarget,
 	createWorkspaceSurfaceTarget,
@@ -31,217 +42,56 @@ type PendingWorkspaceFolderDraft = {
 	depth: number;
 };
 
-type WorkspaceDragTarget = {
-	parentId: string | null;
-	index: number;
-	intent: "before" | "after" | "inside";
-	targetNodeId?: string;
-};
+type WorkspaceDragTarget = NavTreeDropTarget;
 
 type WorkspaceDragState = {
 	draggedNodeId: string;
 	dropTarget: WorkspaceDragTarget | null;
 };
 
-type WorkspaceNodeLocation = {
-	parentId: string | null;
-	index: number;
-	node: WorkspaceTreeNode;
+type PersistedWorkspaceFolderRecord = {
+	id: string;
+	label: string;
+	children?: PersistedWorkspaceFolderRecord[];
+};
+
+type WorkspaceFoldersResponse = {
+	data?: {
+		folders?: PersistedWorkspaceFolderRecord[];
+	};
+	error?: string;
+	message?: string;
 };
 
 const LONG_PRESS_MS = 320;
 
-const createWorkspaceFolderId = (): string => `folder-${Math.random().toString(36).slice(2, 10)}`;
-
 const getWorkspaceTreeNodeId = (node: WorkspaceTreeNode): string => node.id;
 
-const insertWorkspaceFolderNode = (
-	nodes: readonly WorkspaceTreeNode[],
-	parentId: string | null,
-	folder: WorkspaceTreeNode,
-): readonly WorkspaceTreeNode[] => {
-	if (parentId === null) {
-		return [...nodes, folder];
-	}
+const buildPersistedWorkspaceFolderNodes = (
+	folders: readonly PersistedWorkspaceFolderRecord[],
+): WorkspaceTreeNode[] =>
+	folders.map((folder) => ({
+		id: folder.id,
+		label: folder.label,
+		kind: "folder",
+		icon: Folder,
+		children: buildPersistedWorkspaceFolderNodes(folder.children ?? []),
+	}));
 
-	return nodes.map((node) => {
-		if (node.kind !== "folder") {
-			return node;
-		}
+const readPersistedWorkspaceFolders = (body: WorkspaceFoldersResponse): PersistedWorkspaceFolderRecord[] =>
+	Array.isArray(body.data?.folders) ? body.data.folders : [];
 
-		if (node.id === parentId) {
-			return {
+const workspaceTreeAdapter: NavTreeAdapter<WorkspaceTreeNode> = {
+	getNodeId: getWorkspaceTreeNodeId,
+	isBranchNode: (node) => node.kind === "folder",
+	getChildren: (node) => (node.kind === "folder" ? (node.children ?? []) : []),
+	withChildren: (node, children) =>
+		node.kind === "folder"
+			? {
 				...node,
-				children: [...(node.children ?? []), folder],
-			};
-		}
-
-		return {
-			...node,
-			children: node.children ? insertWorkspaceFolderNode(node.children, parentId, folder) : node.children,
-		};
-	});
-};
-
-const findWorkspaceFolderDepth = (nodes: readonly WorkspaceTreeNode[], folderId: string, depth = 0): number | null => {
-	for (const node of nodes) {
-		if (node.kind !== "folder") {
-			continue;
-		}
-
-		if (node.id === folderId) {
-			return depth;
-		}
-
-		const nestedDepth = node.children ? findWorkspaceFolderDepth(node.children, folderId, depth + 1) : null;
-		if (nestedDepth !== null) {
-			return nestedDepth;
-		}
-	}
-
-	return null;
-};
-
-const findWorkspaceNodeLocation = (
-	nodes: readonly WorkspaceTreeNode[],
-	nodeId: string,
-	parentId: string | null = null,
-): WorkspaceNodeLocation | null => {
-	for (let index = 0; index < nodes.length; index += 1) {
-		const node = nodes[index];
-
-		if (node.id === nodeId) {
-			return { parentId, index, node };
-		}
-
-		if (node.kind === "folder" && node.children) {
-			const nestedLocation = findWorkspaceNodeLocation(node.children, nodeId, node.id);
-
-			if (nestedLocation) {
-				return nestedLocation;
+				children: [...children],
 			}
-		}
-	}
-
-	return null;
-};
-
-const workspaceTreeContainsNode = (nodes: readonly WorkspaceTreeNode[], nodeId: string): boolean => {
-	for (const node of nodes) {
-		if (node.id === nodeId) {
-			return true;
-		}
-
-		if (node.kind === "folder" && node.children && workspaceTreeContainsNode(node.children, nodeId)) {
-			return true;
-		}
-	}
-
-	return false;
-};
-
-const removeWorkspaceTreeNode = (
-	nodes: readonly WorkspaceTreeNode[],
-	nodeId: string,
-): { nodes: WorkspaceTreeNode[]; removed: WorkspaceTreeNode | null } => {
-	const nextNodes: WorkspaceTreeNode[] = [];
-	let removed: WorkspaceTreeNode | null = null;
-
-	for (const node of nodes) {
-		if (node.id === nodeId) {
-			removed = node;
-			continue;
-		}
-
-		if (node.kind === "folder" && node.children) {
-			const result = removeWorkspaceTreeNode(node.children, nodeId);
-
-			if (result.removed) {
-				removed = result.removed;
-				nextNodes.push({
-					...node,
-					children: result.nodes,
-				});
-				continue;
-			}
-		}
-
-		nextNodes.push(node);
-	}
-
-	return { nodes: nextNodes, removed };
-};
-
-const insertWorkspaceTreeNode = (
-	nodes: readonly WorkspaceTreeNode[],
-	parentId: string | null,
-	index: number,
-	nodeToInsert: WorkspaceTreeNode,
-): WorkspaceTreeNode[] => {
-	if (parentId === null) {
-		const nextNodes = [...nodes];
-		nextNodes.splice(Math.max(0, Math.min(index, nextNodes.length)), 0, nodeToInsert);
-		return nextNodes;
-	}
-
-	return nodes.map((node) => {
-		if (node.kind !== "folder") {
-			return node;
-		}
-
-		if (node.id === parentId) {
-			const nextChildren = [...(node.children ?? [])];
-			nextChildren.splice(Math.max(0, Math.min(index, nextChildren.length)), 0, nodeToInsert);
-			return {
-				...node,
-				children: nextChildren,
-			};
-		}
-
-		return {
-			...node,
-			children: node.children ? insertWorkspaceTreeNode(node.children, parentId, index, nodeToInsert) : node.children,
-		};
-	});
-};
-
-const moveWorkspaceTreeNode = (
-	nodes: readonly WorkspaceTreeNode[],
-	draggedNodeId: string,
-	dropTarget: WorkspaceDragTarget,
-): WorkspaceTreeNode[] => {
-	const location = findWorkspaceNodeLocation(nodes, draggedNodeId);
-
-	if (!location) {
-		return [...nodes];
-	}
-
-	if (
-		location.node.kind === "folder" &&
-		dropTarget.parentId !== null &&
-		((location.node.children && workspaceTreeContainsNode(location.node.children, dropTarget.parentId)) ||
-			dropTarget.parentId === location.node.id)
-	) {
-		return [...nodes];
-	}
-
-	let normalizedIndex = dropTarget.index;
-
-	if (dropTarget.parentId === location.parentId && dropTarget.index > location.index) {
-		normalizedIndex -= 1;
-	}
-
-	if (dropTarget.parentId === location.parentId && normalizedIndex === location.index) {
-		return [...nodes];
-	}
-
-	const removalResult = removeWorkspaceTreeNode(nodes, draggedNodeId);
-
-	if (!removalResult.removed) {
-		return [...nodes];
-	}
-
-	return insertWorkspaceTreeNode(removalResult.nodes, dropTarget.parentId, normalizedIndex, removalResult.removed);
+			: node,
 };
 
 const FolderDraftRow = (props: {
@@ -292,10 +142,6 @@ const WorkspaceHomeEntry = (props: {
 	item: WorkspaceStaticItem;
 	onOpenContextMenu: (event: MouseEvent, target: WorkspaceContextMenuTarget) => void;
 	onOpenContextMenuFromKeyboard: (element: HTMLElement, target: WorkspaceContextMenuTarget) => void;
-	onNodePointerDown: (event: PointerEvent, nodeId: string) => void;
-	onNodePointerMove: (event: PointerEvent, parentId: string | null, index: number, node: WorkspaceTreeNode) => void;
-	dragState: WorkspaceDragState | null;
-	isTreeClickSuppressed: () => boolean;
 }): JSX.Element => {
 	const Icon = props.item.icon;
 	const target = createWorkspaceStaticTarget(props.item);
@@ -350,17 +196,21 @@ const WorkspaceTreeBranch = (props: {
 	onCancelPendingFolder: () => void;
 	onOpenContextMenu: (event: MouseEvent, target: WorkspaceContextMenuTarget) => void;
 	onOpenContextMenuFromKeyboard: (element: HTMLElement, target: WorkspaceContextMenuTarget) => void;
+	onNodePointerDown: (event: PointerEvent, nodeId: string) => void;
+	onNodePointerMove: (event: PointerEvent, parentId: string | null, index: number, node: WorkspaceTreeNode) => void;
+	dragState: WorkspaceDragState | null;
+	isTreeClickSuppressed: () => boolean;
 }): JSX.Element => {
 	const depth = () => props.depth ?? 0;
 	const parentId = () => props.parentId ?? null;
 
 	return (
 		<ul class={styles.treeList} role="list">
-		<Show when={props.nodes.length === 0 && props.pendingFolderDraft?.parentId !== parentId()}>
-			<li>
-				<div class={styles.treeEmptySlot} style={{ "--tree-depth": String(depth()) }} />
-			</li>
-		</Show>
+			<Show when={props.nodes.length === 0 && props.pendingFolderDraft?.parentId !== parentId()}>
+				<li>
+					<div class={styles.treeEmptySlot} style={{ "--tree-depth": String(depth()) }} />
+				</li>
+			</Show>
 			<For each={props.nodes}>
 				{(node, indexAccessor): JSX.Element => {
 					const Icon = getWorkspaceNodeIcon(node);
@@ -485,24 +335,105 @@ const WorkspaceTreeBranch = (props: {
 
 export const WorkspaceSidebar = (props: WorkspaceSidebarProps): JSX.Element => {
 	const appShellData = useAppShellData();
+	const activeProject = () => appShellData.activeProject();
 	const [isProjectDrawerOpen, setIsProjectDrawerOpen] = createSignal(false);
 	const [workspaceTreeNodes, setWorkspaceTreeNodes] = createSignal<readonly WorkspaceTreeNode[]>(appShellData.workspaceTree());
+	const [persistedFolders, setPersistedFolders] = createSignal<readonly PersistedWorkspaceFolderRecord[]>([]);
 	const [collapsedFolderIds, setCollapsedFolderIds] = createSignal<readonly string[]>([]);
 	const [pendingFolderDraft, setPendingFolderDraft] = createSignal<PendingWorkspaceFolderDraft | null>(null);
 	const [pendingFolderName, setPendingFolderName] = createSignal("");
 	const [dragState, setDragState] = createSignal<WorkspaceDragState | null>(null);
 	const [suppressNextTreeClick, setSuppressNextTreeClick] = createSignal(false);
+	let lastSelectedProjectId: string | null = null;
+	let latestPersistedFoldersRequest = 0;
 	const contextMenu = createWorkspaceContextMenuController();
 	let longPressTimer: number | undefined;
 	let suppressClickTimer: number | undefined;
 	const railToggleLabel = (): string => (props.railCollapsed ? "Expand server rail" : "Collapse server rail");
 	const sidebarContextMenuTarget = createWorkspaceSurfaceTarget(appShellData.activeProject());
 	const isFolderCollapsed = (folderId: string): boolean => collapsedFolderIds().includes(folderId);
+	const folderIds = (): string[] => collectBranchNodeIds(workspaceTreeNodes(), workspaceTreeAdapter);
+	const totalFolderCount = (): number => folderIds().length;
+	const areAllFoldersCollapsed = (): boolean => {
+		const count = totalFolderCount();
+		return count > 0 && collapsedFolderIds().length >= count;
+	};
+	const workspaceFolderToggleLabel = (): string =>
+		areAllFoldersCollapsed() ? "Expand all folders" : "Collapse all folders";
 	const toggleFolder = (folderId: string): void => {
 		setCollapsedFolderIds((current) =>
 			current.includes(folderId) ? current.filter((id) => id !== folderId) : [...current, folderId],
 		);
 	};
+	const expandAllFolders = (): void => {
+		setCollapsedFolderIds([]);
+	};
+	const collapseAllFolders = (): void => {
+		setCollapsedFolderIds(folderIds());
+	};
+	const toggleAllFolders = (): void => {
+		if (areAllFoldersCollapsed()) {
+			expandAllFolders();
+			return;
+		}
+
+		collapseAllFolders();
+	};
+	const resetWorkspaceTreeInteractionState = (): void => {
+		setCollapsedFolderIds([]);
+		setPendingFolderDraft(null);
+		setPendingFolderName("");
+		setDragState(null);
+	};
+	const syncWorkspaceTree = (): void => {
+		const nextTree = activeProject()?.id
+			? buildPersistedWorkspaceFolderNodes(persistedFolders())
+			: appShellData.workspaceTree();
+		const availableFolderIds = new Set(collectBranchNodeIds(nextTree, workspaceTreeAdapter));
+
+		setWorkspaceTreeNodes(nextTree);
+		setCollapsedFolderIds((current) => current.filter((id) => availableFolderIds.has(id)));
+	};
+	const loadPersistedFolders = async (projectId: string): Promise<void> => {
+		const requestId = latestPersistedFoldersRequest + 1;
+		latestPersistedFoldersRequest = requestId;
+
+		if (!projectId) {
+			setPersistedFolders([]);
+			return;
+		}
+
+		if (!isUuidString(projectId)) {
+			setPersistedFolders([]);
+			return;
+		}
+
+		try {
+			const response = await fetch(`${resolveAPIBase()}/projects/${projectId}/tree/folders`, {
+				headers: {
+					Accept: "application/json",
+				},
+			});
+			const body = (await response.json()) as WorkspaceFoldersResponse;
+
+			if (requestId !== latestPersistedFoldersRequest) {
+				return;
+			}
+
+			if (!response.ok) {
+				throw new Error(body.message || "Failed to load project tree folders.");
+			}
+
+			setPersistedFolders(readPersistedWorkspaceFolders(body));
+		} catch (error) {
+			if (requestId !== latestPersistedFoldersRequest) {
+				return;
+			}
+
+			console.error(error);
+			setPersistedFolders([]);
+		}
+	};
 	const clearLongPressTimer = (): void => {
 		if (longPressTimer !== undefined) {
 			window.clearTimeout(longPressTimer);
@@ -523,11 +454,27 @@ export const WorkspaceSidebar = (props: WorkspaceSidebarProps): JSX.Element => {
 	};
 
 	createEffect(() => {
-		setWorkspaceTreeNodes(appShellData.workspaceTree());
-		setCollapsedFolderIds([]);
-		setPendingFolderDraft(null);
-		setPendingFolderName("");
-		setDragState(null);
+		syncWorkspaceTree();
+	});
+
+	createEffect(() => {
+		const projectId = activeProject()?.id ?? null;
+
+		if (lastSelectedProjectId === null) {
+			lastSelectedProjectId = projectId;
+			return;
+		}
+
+		if (projectId === lastSelectedProjectId) {
+			return;
+		}
+
+		lastSelectedProjectId = projectId;
+		resetWorkspaceTreeInteractionState();
+	});
+
+	createEffect(() => {
+		void loadPersistedFolders(activeProject()?.id ?? "");
 	});
 
 	onMount(() => {
@@ -546,7 +493,7 @@ export const WorkspaceSidebar = (props: WorkspaceSidebarProps): JSX.Element => {
 
 			suppressTreeClickTemporarily();
 			setWorkspaceTreeNodes((current) =>
-				moveWorkspaceTreeNode(current, nextDragState.draggedNodeId, nextDragState.dropTarget as WorkspaceDragTarget),
+				moveTreeNode(current, nextDragState.draggedNodeId, nextDragState.dropTarget as WorkspaceDragTarget, workspaceTreeAdapter),
 			);
 			setDragState(null);
 		};
@@ -587,9 +534,10 @@ export const WorkspaceSidebar = (props: WorkspaceSidebarProps): JSX.Element => {
 		setPendingFolderDraft({ parentId, depth });
 	};
 
-	const submitPendingFolder = (): void => {
+	const submitPendingFolder = async (): Promise<void> => {
 		const name = pendingFolderName().trim();
 		const draft = pendingFolderDraft();
+		const projectId = activeProject()?.id ?? "";
 
 		if (!draft) {
 			return;
@@ -601,17 +549,36 @@ export const WorkspaceSidebar = (props: WorkspaceSidebarProps): JSX.Element => {
 			return;
 		}
 
-		setWorkspaceTreeNodes((current) =>
-			insertWorkspaceFolderNode(current, draft.parentId, {
-				id: createWorkspaceFolderId(),
-				label: name,
-				kind: "folder",
-				icon: Folder,
-				children: [],
-			}),
-		);
-		setPendingFolderDraft(null);
-		setPendingFolderName("");
+		if (!projectId || !isUuidString(projectId)) {
+			cancelPendingFolder();
+			return;
+		}
+
+		try {
+			const response = await fetch(`${resolveAPIBase()}/projects/${projectId}/tree/folders`, {
+				method: "POST",
+				headers: {
+					Accept: "application/json",
+					"Content-Type": "application/json",
+				},
+				body: JSON.stringify({
+					name,
+					parentFolderId: draft.parentId,
+				}),
+			});
+
+			const body = (await response.json()) as WorkspaceFoldersResponse;
+
+			if (!response.ok) {
+				throw new Error(body.message || "Failed to create project tree folder.");
+			}
+
+			setPersistedFolders(readPersistedWorkspaceFolders(body));
+			setPendingFolderDraft(null);
+			setPendingFolderName("");
+		} catch (error) {
+			console.error(error);
+		}
 	};
 
 	const cancelPendingFolder = (): void => {
@@ -619,6 +586,17 @@ export const WorkspaceSidebar = (props: WorkspaceSidebarProps): JSX.Element => {
 		setPendingFolderName("");
 	};
 
+	const handleHeaderActionClick = (actionId: string): void => {
+		switch (actionId) {
+			case "toggle-workspace-folders":
+				toggleAllFolders();
+				return;
+			case "search-workspace":
+			default:
+				return;
+		}
+	};
+
 	const handleNodePointerDown = (event: PointerEvent, nodeId: string): void => {
 		if (event.button !== 0 || pendingFolderDraft()) {
 			return;
@@ -643,33 +621,21 @@ export const WorkspaceSidebar = (props: WorkspaceSidebarProps): JSX.Element => {
 			return;
 		}
 
-		const bounds = event.currentTarget.getBoundingClientRect();
-		const relativeY = bounds.height <= 0 ? 0.5 : (event.clientY - bounds.top) / bounds.height;
-		let nextTarget: WorkspaceDragTarget;
-
-		if (node.kind === "folder") {
-			if (relativeY < 0.28) {
-				nextTarget = { parentId, index, intent: "before", targetNodeId: node.id };
-			} else if (relativeY > 0.72) {
-				nextTarget = { parentId, index: index + 1, intent: "after", targetNodeId: node.id };
-			} else {
-				nextTarget = {
-					parentId: node.id,
-					index: (node.children ?? []).length,
-					intent: "inside",
-					targetNodeId: node.id,
-				};
-			}
-		} else {
-			nextTarget = {
-				parentId,
-				index: relativeY < 0.5 ? index : index + 1,
-				intent: relativeY < 0.5 ? "before" : "after",
-				targetNodeId: node.id,
-			};
+		const relativeY = getPointerRelativeY(event);
+		if (relativeY === null) {
+			return;
 		}
 
-		setDragState({ ...nextDragState, dropTarget: nextTarget });
+		setDragState({
+			...nextDragState,
+			dropTarget: resolveTreeDropTarget({
+				parentId,
+				index,
+				node,
+				relativeY,
+				adapter: workspaceTreeAdapter,
+			}),
+		});
 	};
 
 	const handleContextActionSelect = (action: WorkspaceContextMenuAction, target: WorkspaceContextMenuTarget): void => {
@@ -677,13 +643,13 @@ export const WorkspaceSidebar = (props: WorkspaceSidebarProps): JSX.Element => {
 			return;
 		}
 
-		switch (target.kind) {
+			switch (target.kind) {
 			case "workspace":
 			case "home":
 				beginFolderDraft(null, 0);
 				return;
 			case "folder":
-				beginFolderDraft(target.id, (findWorkspaceFolderDepth(workspaceTreeNodes(), target.id) ?? 0) + 1);
+				beginFolderDraft(target.id, (findTreeNodeDepth(workspaceTreeNodes(), target.id, workspaceTreeAdapter) ?? 0) + 1);
 				return;
 			case "settings":
 			case "item":
@@ -729,12 +695,30 @@ export const WorkspaceSidebar = (props: WorkspaceSidebarProps): JSX.Element => {
 							{props.railCollapsed ? <ChevronRight size={16} strokeWidth={2} /> : <ChevronLeft size={16} strokeWidth={2} />}
 						</button>
 
-						<For each={workspaceSidebarHeaderActions}>
-							{(action): JSX.Element => {
-								const Icon = action.icon;
+					<For each={workspaceSidebarHeaderActions}>
+						{(action): JSX.Element => {
+							const label =
+								action.id === "toggle-workspace-folders"
+										? workspaceFolderToggleLabel()
+										: action.label;
+							const Icon =
+								action.id === "toggle-workspace-folders"
+										? areAllFoldersCollapsed()
+											? UnfoldVertical
+											: ListCollapse
+										: action.icon;
 
-								return (
-									<button type="button" class={styles.headerActionButton} aria-label={action.label} title={action.label} data-slot="workspace-sidebar-header-action" data-action-id={action.id}>
+							return (
+								<button
+									type="button"
+									class={styles.headerActionButton}
+									aria-label={label}
+									title={label}
+									data-slot="workspace-sidebar-header-action"
+									data-action-id={action.id}
+									disabled={action.id === "toggle-workspace-folders" && totalFolderCount() === 0}
+									onClick={(): void => handleHeaderActionClick(action.id)}
+									>
 										<Icon size={16} strokeWidth={2} />
 									</button>
 								);
@@ -794,13 +778,13 @@ export const WorkspaceSidebar = (props: WorkspaceSidebarProps): JSX.Element => {
 								onPendingFolderNameChange={setPendingFolderName}
 								onSubmitPendingFolder={submitPendingFolder}
 								onCancelPendingFolder={cancelPendingFolder}
-								onOpenContextMenu={contextMenu.openMenu}
-								onOpenContextMenuFromKeyboard={contextMenu.openMenuFromElement}
-								onNodePointerDown={handleNodePointerDown}
-								onNodePointerMove={handleNodePointerMove}
-								dragState={dragState()}
-								isTreeClickSuppressed={suppressNextTreeClick}
-							/>
+									onOpenContextMenu={contextMenu.openMenu}
+									onOpenContextMenuFromKeyboard={contextMenu.openMenuFromElement}
+									onNodePointerDown={handleNodePointerDown}
+									onNodePointerMove={handleNodePointerMove}
+									dragState={dragState()}
+									isTreeClickSuppressed={suppressNextTreeClick}
+								/>
 						</div>
 					</div>
 				</div>

Frontend/src/components/shell/data/shell.data.ts

@@ -9,6 +9,7 @@ import {
 	Home,
 	Keyboard,
 	LayoutGrid,
+	ListCollapse,
 	LogOut,
 	Repeat,
 	Search,
@@ -476,6 +477,7 @@ export const workspaceTree: readonly WorkspaceTreeNode[] = [
 
 export const workspaceSidebarHeaderActions: readonly SidebarHeaderAction[] = [
 	{ id: "search-workspace", label: "Search workspace", icon: Search },
+	{ id: "toggle-workspace-folders", label: "Collapse all folders", icon: ListCollapse },
 ] as const;
 
 export const mobileBottomNavItems: readonly MobileBottomNavItem[] = [

Frontend/src/components/shell/shared/_tree-nav.scss

@@ -0,0 +1,188 @@
+@use "../../../styles/tools/mixins" as *;
+
+@mixin section-label {
+	@include text-caption;
+	color: var(--color-text-subtle);
+	text-transform: uppercase;
+	letter-spacing: 0.04em;
+}
+
+@mixin tree-list {
+	list-style: none;
+	display: grid;
+	gap: var(--space-1);
+	padding: 0;
+}
+
+@mixin empty-slot {
+	min-height: calc(var(--control-size-lg) - var(--space-2));
+	padding-left: calc(var(--space-3) + (var(--tree-depth, 0) * var(--space-4)));
+	border-radius: var(--radius-lg);
+	border: 1px dashed color-mix(in srgb, var(--color-border) 38%, transparent);
+	opacity: 0.35;
+}
+
+@mixin input-row {
+	width: 100%;
+	min-width: 0;
+	display: grid;
+	grid-template-columns: auto minmax(0, 1fr);
+	align-items: center;
+	gap: var(--space-2);
+	min-height: calc(var(--control-size-lg) - var(--space-2));
+	padding: var(--space-2) var(--space-3);
+	padding-left: calc(var(--space-3) + (var(--tree-depth, 0) * var(--space-4)));
+	border: 1px solid color-mix(in srgb, var(--color-border) 42%, transparent);
+	border-radius: var(--radius-lg);
+	background: color-mix(in srgb, var(--color-surface) 94%, transparent);
+}
+
+@mixin input {
+	width: 100%;
+	min-width: 0;
+	border: 0;
+	background: transparent;
+	color: var(--color-text);
+	font: inherit;
+	outline: none;
+
+	&::placeholder {
+		color: var(--color-text-muted);
+	}
+}
+
+@mixin item {
+	width: 100%;
+	min-width: 0;
+	display: grid;
+	position: relative;
+	isolation: isolate;
+	grid-template-columns: auto auto minmax(0, 1fr) auto;
+	align-items: center;
+	gap: var(--space-2);
+	min-height: calc(var(--control-size-lg) - var(--space-2));
+	padding: var(--space-2) var(--space-3);
+	padding-left: calc(var(--space-3) + (var(--tree-depth, 0) * var(--space-4)));
+	border: 0;
+	border-radius: 0;
+	background: transparent;
+	color: var(--color-text-muted);
+	text-align: left;
+	transition:
+		color 160ms var(--easing-standard),
+		box-shadow 160ms var(--easing-standard),
+		transform 180ms var(--easing-standard);
+
+	&::after {
+		content: "";
+		position: absolute;
+		inset: 0;
+		border: 1px solid transparent;
+		border-radius: var(--radius-lg);
+		background: transparent;
+		transition:
+			background 160ms var(--easing-standard),
+			border-color 160ms var(--easing-standard),
+			box-shadow 160ms var(--easing-standard);
+		pointer-events: none;
+		z-index: 0;
+	}
+
+	> * {
+		position: relative;
+		z-index: 1;
+	}
+}
+
+@mixin item-hover {
+	color: var(--color-text);
+
+	&::after {
+		background: color-mix(in srgb, var(--color-surface-hover) 80%, var(--color-accent-soft) 20%);
+		box-shadow: inset 0 1px 0 color-mix(in srgb, white 4%, transparent);
+	}
+}
+
+@mixin item-folder {
+	color: var(--color-text);
+}
+
+@mixin item-dragging {
+	opacity: 0.45;
+	transform: scale(0.985);
+	box-shadow: none;
+}
+
+@mixin item-drop-before {
+	&::before {
+		content: "";
+		position: absolute;
+		left: calc(var(--space-3) + (var(--tree-depth, 0) * var(--space-4)));
+		right: var(--space-3);
+		top: calc((var(--space-1) * -0.5) - 1px);
+		height: 2px;
+		border-radius: 999px;
+		background: color-mix(in srgb, var(--color-accent-strong) 78%, transparent);
+		pointer-events: none;
+		z-index: 2;
+	}
+}
+
+@mixin item-drop-after {
+	&::before {
+		content: "";
+		position: absolute;
+		left: calc(var(--space-3) + (var(--tree-depth, 0) * var(--space-4)));
+		right: var(--space-3);
+		bottom: calc((var(--space-1) * -0.5) - 1px);
+		height: 2px;
+		border-radius: 999px;
+		background: color-mix(in srgb, var(--color-accent-strong) 78%, transparent);
+		pointer-events: none;
+		z-index: 2;
+	}
+}
+
+@mixin item-drop-inside {
+	color: var(--color-text);
+
+	&::after {
+		border-color: color-mix(in srgb, var(--color-accent-strong) 55%, transparent);
+		background: color-mix(in srgb, var(--color-accent-soft) 36%, var(--color-surface));
+		box-shadow: inset 0 1px 0 color-mix(in srgb, white 4%, transparent);
+	}
+}
+
+@mixin folder-chevron {
+	color: var(--color-text-muted);
+	transition: transform 160ms var(--easing-standard);
+}
+
+@mixin folder-chevron-open {
+	transform: rotate(90deg);
+}
+
+@mixin item-active {
+	color: var(--color-text);
+
+	&::after {
+		border-color: var(--color-border);
+		background: var(--color-surface);
+		box-shadow: inset 0 1px 0 color-mix(in srgb, white 4%, transparent);
+	}
+}
+
+@mixin icon {
+	color: inherit;
+	opacity: 0.85;
+}
+
+@mixin label {
+	@include text-label;
+	min-width: 0;
+}
+
+@mixin item-meta {
+	@include text-caption;
+	color: var(--color-text-muted);
+}

Frontend/src/components/shell/shared/navTreeDnd.ts

@@ -0,0 +1,278 @@
+export type NavTreeDropIntent = "before" | "after" | "inside";
+
+export type NavTreeDropTarget = {
+	parentId: string | null;
+	index: number;
+	intent: NavTreeDropIntent;
+	targetNodeId?: string;
+};
+
+export type NavTreeDragState = {
+	draggedNodeId: string;
+	dropTarget: NavTreeDropTarget | null;
+};
+
+export type NavTreeLocation<TNode> = {
+	parentId: string | null;
+	index: number;
+	node: TNode;
+};
+
+export type NavTreeAdapter<TNode> = {
+	getNodeId: (node: TNode) => string;
+	isBranchNode: (node: TNode) => boolean;
+	getChildren: (node: TNode) => readonly TNode[];
+	withChildren: (node: TNode, children: readonly TNode[]) => TNode;
+};
+
+const UUID_PATTERN = /^[0-9a-f]{8}-[0-9a-f]{4}-[1-5][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i;
+
+export const isUuidString = (value: string | null | undefined): boolean => {
+	if (typeof value !== "string") {
+		return false;
+	}
+
+	return UUID_PATTERN.test(value.trim());
+};
+
+export const collectBranchNodeIds = <TNode>(
+	nodes: readonly TNode[],
+	adapter: NavTreeAdapter<TNode>,
+): string[] => {
+	const ids: string[] = [];
+
+	for (const node of nodes) {
+		if (!adapter.isBranchNode(node)) {
+			continue;
+		}
+
+		ids.push(adapter.getNodeId(node));
+		ids.push(...collectBranchNodeIds(adapter.getChildren(node), adapter));
+	}
+
+	return ids;
+};
+
+export const findTreeNodeLocation = <TNode>(
+	nodes: readonly TNode[],
+	nodeId: string,
+	adapter: NavTreeAdapter<TNode>,
+	parentId: string | null = null,
+): NavTreeLocation<TNode> | null => {
+	for (let index = 0; index < nodes.length; index += 1) {
+		const node = nodes[index];
+
+		if (adapter.getNodeId(node) === nodeId) {
+			return { parentId, index, node };
+		}
+
+		if (!adapter.isBranchNode(node)) {
+			continue;
+		}
+
+		const nestedLocation = findTreeNodeLocation(adapter.getChildren(node), nodeId, adapter, adapter.getNodeId(node));
+		if (nestedLocation) {
+			return nestedLocation;
+		}
+	}
+
+	return null;
+};
+
+export const findTreeNodeDepth = <TNode>(
+	nodes: readonly TNode[],
+	nodeId: string,
+	adapter: NavTreeAdapter<TNode>,
+	depth = 0,
+): number | null => {
+	for (const node of nodes) {
+		if (adapter.getNodeId(node) === nodeId) {
+			return depth;
+		}
+
+		if (!adapter.isBranchNode(node)) {
+			continue;
+		}
+
+		const nestedDepth = findTreeNodeDepth(adapter.getChildren(node), nodeId, adapter, depth + 1);
+		if (nestedDepth !== null) {
+			return nestedDepth;
+		}
+	}
+
+	return null;
+};
+
+export const treeContainsNode = <TNode>(
+	nodes: readonly TNode[],
+	nodeId: string,
+	adapter: NavTreeAdapter<TNode>,
+): boolean => {
+	for (const node of nodes) {
+		if (adapter.getNodeId(node) === nodeId) {
+			return true;
+		}
+
+		if (adapter.isBranchNode(node) && treeContainsNode(adapter.getChildren(node), nodeId, adapter)) {
+			return true;
+		}
+	}
+
+	return false;
+};
+
+export const removeTreeNode = <TNode>(
+	nodes: readonly TNode[],
+	nodeId: string,
+	adapter: NavTreeAdapter<TNode>,
+): { nodes: TNode[]; removed: TNode | null } => {
+	const nextNodes: TNode[] = [];
+	let removed: TNode | null = null;
+
+	for (const node of nodes) {
+		if (adapter.getNodeId(node) === nodeId) {
+			removed = node;
+			continue;
+		}
+
+		if (adapter.isBranchNode(node)) {
+			const result = removeTreeNode(adapter.getChildren(node), nodeId, adapter);
+
+			if (result.removed) {
+				removed = result.removed;
+				nextNodes.push(adapter.withChildren(node, result.nodes));
+				continue;
+			}
+		}
+
+		nextNodes.push(node);
+	}
+
+	return { nodes: nextNodes, removed };
+};
+
+export const insertTreeNode = <TNode>(
+	nodes: readonly TNode[],
+	parentId: string | null,
+	index: number,
+	nodeToInsert: TNode,
+	adapter: NavTreeAdapter<TNode>,
+): TNode[] => {
+	if (parentId === null) {
+		const nextNodes = [...nodes];
+		nextNodes.splice(Math.max(0, Math.min(index, nextNodes.length)), 0, nodeToInsert);
+		return nextNodes;
+	}
+
+	return nodes.map((node) => {
+		if (!adapter.isBranchNode(node)) {
+			return node;
+		}
+
+		if (adapter.getNodeId(node) === parentId) {
+			const nextChildren = [...adapter.getChildren(node)];
+			nextChildren.splice(Math.max(0, Math.min(index, nextChildren.length)), 0, nodeToInsert);
+			return adapter.withChildren(node, nextChildren);
+		}
+
+		return adapter.withChildren(node, insertTreeNode(adapter.getChildren(node), parentId, index, nodeToInsert, adapter));
+	});
+};
+
+export const moveTreeNode = <TNode>(
+	nodes: readonly TNode[],
+	draggedNodeId: string,
+	dropTarget: NavTreeDropTarget,
+	adapter: NavTreeAdapter<TNode>,
+): TNode[] => {
+	const location = findTreeNodeLocation(nodes, draggedNodeId, adapter);
+
+	if (!location) {
+		return [...nodes];
+	}
+
+	if (
+		adapter.isBranchNode(location.node) &&
+		dropTarget.parentId !== null &&
+		(treeContainsNode(adapter.getChildren(location.node), dropTarget.parentId, adapter) ||
+			dropTarget.parentId === adapter.getNodeId(location.node))
+	) {
+		return [...nodes];
+	}
+
+	let normalizedIndex = dropTarget.index;
+	if (dropTarget.parentId === location.parentId && dropTarget.index > location.index) {
+		normalizedIndex -= 1;
+	}
+
+	if (dropTarget.parentId === location.parentId && normalizedIndex === location.index) {
+		return [...nodes];
+	}
+
+	const removalResult = removeTreeNode(nodes, draggedNodeId, adapter);
+	if (!removalResult.removed) {
+		return [...nodes];
+	}
+
+	return insertTreeNode(removalResult.nodes, dropTarget.parentId, normalizedIndex, removalResult.removed, adapter);
+};
+
+export const getPointerRelativeY = (event: PointerEvent): number | null => {
+	const currentTarget = event.currentTarget;
+	if (!(currentTarget instanceof HTMLElement)) {
+		return null;
+	}
+
+	const bounds = currentTarget.getBoundingClientRect();
+	return bounds.height <= 0 ? 0.5 : (event.clientY - bounds.top) / bounds.height;
+};
+
+export const resolveTreeDropTarget = <TNode>(params: {
+	parentId: string | null;
+	index: number;
+	node: TNode;
+	relativeY: number;
+	adapter: NavTreeAdapter<TNode>;
+	beforeThreshold?: number;
+	beforeThresholdFirstSibling?: number;
+	afterThreshold?: number;
+}): NavTreeDropTarget => {
+	const {
+		parentId,
+		index,
+		node,
+		relativeY,
+		adapter,
+		beforeThreshold = 0.28,
+		beforeThresholdFirstSibling = 0.42,
+		afterThreshold = 0.72,
+	} = params;
+
+	const targetNodeId = adapter.getNodeId(node);
+
+	if (adapter.isBranchNode(node)) {
+		const nextBeforeThreshold = index === 0 ? beforeThresholdFirstSibling : beforeThreshold;
+
+		if (relativeY < nextBeforeThreshold) {
+			return { parentId, index, intent: "before", targetNodeId };
+		}
+
+		if (relativeY > afterThreshold) {
+			return { parentId, index: index + 1, intent: "after", targetNodeId };
+		}
+
+		return {
+			parentId: targetNodeId,
+			index: adapter.getChildren(node).length,
+			intent: "inside",
+			targetNodeId,
+		};
+	}
+
+	return {
+		parentId,
+		index: relativeY < 0.5 ? index : index + 1,
+		intent: relativeY < 0.5 ? "before" : "after",
+		targetNodeId,
+	};
+};