feat(ui): add an accessible VS Code-parity Tree

.storybook/main.ts

@@ -6,7 +6,11 @@ import type { StorybookConfig } from "@storybook/react-vite";
 
 const config: StorybookConfig = {
 	stories: ["../packages/*/src/**/*.stories.@(ts|tsx)"],
-	addons: ["@storybook/addon-a11y", "@storybook/addon-docs"],
+	addons: [
+		"@storybook/addon-a11y",
+		"@storybook/addon-docs",
+		"storybook-addon-pseudo-states",
+	],
 	framework: {
 		name: "@storybook/react-vite",
 		options: {},

AGENTS.md

@@ -94,6 +94,15 @@ Non-negotiables:
 - Extension panels must call **both** `buildCommandHandlers` and
   `buildRequestHandlers` (empty `{}` is fine). This gives a compile error
   when anyone adds an action to the API without a matching handler.
+- Every webview and Storybook build runs the React Compiler, so components
+  and hooks must follow the rules of React: no reading or writing a ref
+  during render, no mutating props, state, or anything already rendered,
+  and hooks called unconditionally. A component that breaks them is skipped
+  silently and loses its memoization. Parameter defaults that read another
+  prop (`focused = adapter?.focusedId === row.node.id`) are the usual
+  culprit; put those defaults in the body. `useMemo` and `useCallback` are
+  rarely needed, and when kept they must list every dependency, or
+  `react-hooks/preserve-manual-memoization` fails the lint.
 
 ## Code Style
 

package.json

@@ -813,6 +813,7 @@
 		"@tanstack/react-query": "catalog:",
 		"@testing-library/jest-dom": "^7.0.1",
 		"@testing-library/react": "^16.3.2",
+		"@testing-library/user-event": "catalog:",
 		"@tsconfig/node22": "^22.0.5",
 		"@types/mocha": "^10.0.10",
 		"@types/node": "^22.20.1",
@@ -856,6 +857,7 @@
 		"react": "catalog:",
 		"react-dom": "catalog:",
 		"storybook": "catalog:",
+		"storybook-addon-pseudo-states": "catalog:",
 		"typescript": "catalog:",
 		"typescript-eslint": "^8.66.0",
 		"utf-8-validate": "^6.0.6",

packages/ui/README.md

@@ -7,6 +7,11 @@ Its stable separation boundary is the public root exports, no monorepo runtime
 imports, and component CSS using only semantic `--ui-*` tokens. A future package
 build can emit those same entry points without API changes.
 
+Consumers compile these components with the React Compiler, so they follow the
+rules of React and lean on it for memoization. A component that breaks the
+rules is skipped silently rather than reported, which for a list or a tree
+costs a re-render per row, so check with the compiler and not only the linter.
+
 ## CSS
 
 Import the semantic token mapping and codicon assets once in each real webview
@@ -38,12 +43,90 @@ Every component forwards `className` and `style` to its root element, and
 default rules use single-class specificity, so a consumer class imported
 after the library overrides any default (width, height, spacing).
 
-Where VS Code's stable rendering and its Modern UI preview
-(`workbench.experimental.modernUI`) diverge, components follow Modern UI,
-and new components should too. Webviews get no signal for the setting, so
-the default cannot follow the host. Until the design settles,
-`data-ui-style="stable"` on the document root restores the stable-parity
-menu motion; Storybook's "UI style" toolbar switch toggles it live.
+VS Code currently uses its stable UI by default; Modern UI remains behind the
+experimental `workbench.experimental.modernUI` setting. `@repo/ui`
+intentionally uses Modern UI as its package default because webviews receive no
+host signal for that setting. The divergence is isolated: set
+`data-ui-style="stable"` on the document root to restore stable row geometry,
+focus behavior, and menu motion. Storybook's "UI style" toolbar switch toggles
+that override live.
+
+## Tree
+
+`Tree` is controlled: `nodes` describe the hierarchy, `expandedIds` controls
+branches, and `selectedItemId` controls selection. Each
+visible node renders as a flat `treeitem`, while normal keyboard navigation
+keeps DOM focus on the `tree` container and identifies the active row with
+`aria-activedescendant`. Focus and selection are independent.
+
+```tsx
+const [selectedItemId, setSelectedItemId] = useState("src");
+const [expandedIds, setExpandedIds] = useState<readonly string[]>(["src"]);
+
+<Tree
+	aria-label="Explorer"
+	variant="explorer"
+	nodes={[
+		{
+			id: "src",
+			label: "src",
+			children: [{ id: "tree", label: "Tree.tsx", icon: "symbol-class" }],
+		},
+		{ id: "readme", label: "README.md", icon: "markdown" },
+	]}
+	expandedIds={expandedIds}
+	onExpandedIdsChange={setExpandedIds}
+	selectedItemId={selectedItemId}
+	onSelectedItemChange={setSelectedItemId}
+/>;
+```
+
+Ids must be unique across the whole tree. A string `label` is also the
+accessible name and type-navigation value; a rich label must provide
+`textValue`. `children` marks a branch, including an empty array for a branch
+whose children are still loading. `icon`, `action`, and `className` customize
+the row. Actions stay live on plain hover, as in the native list, and are
+isolated from row selection and expansion.
+
+Arrow Up/Down, Home, and End move the active row through visible rows. Arrow Right
+expands a branch or enters it; Arrow Left collapses it or moves to its parent.
+
+`expandMode="singleClick"` is the default: clicking a branch selects
+and toggles it, and Enter does the same. With `expandMode="doubleClick"`, a
+single click or Enter only selects and a double click toggles expansion. Space
+toggles a branch without selecting it, or selects a leaf. A normal-row twistie
+toggles without changing selection. Alt-click recursively toggles descendant
+branches.
+
+Escape clears selection, then the active focus mark. Once neither remains,
+Escape is left to the host. The root `onKeyDown` runs first, so a host
+can intercept shortcuts with `preventDefault()`.
+
+```mermaid
+flowchart LR
+  accTitle: Tree architecture
+  accDescr: Data and input flow through the pure Tree modules into the React and DOM adapter.
+
+  Props[Nodes and controlled props] --> Model[treeModel.ts]
+  Events[Pointer and keyboard events] --> Policy[treePolicy.ts]
+  Policy --> Commands[Tree commands]
+  Model --> Transition[treeTransition.ts]
+  Commands --> Transition
+  Transition --> Adapter[useTreeAdapter.ts]
+  Adapter --> Rows[Tree.tsx and TreeRow.tsx]
+```
+
+The model, policy, and transitions stay pure. The adapter owns React and DOM
+integration. The flat visible model supports future windowing, but the Tree is
+not currently virtualized.
+
+Rows are 22px tall and keep the VS Code twistie gutter. For Explorer-style file
+trees whose branches have no icons, `variant="explorer"` aligns leaf icons with
+branch twisties; do not combine it with branch icons. Indent guides appear on
+hover, selected ancestor paths stay active, and the focused path is active only
+while the tree has focus. The package default uses inset Modern UI rows;
+`data-ui-style="stable"` restores edge-to-edge square rows and stable focus
+styling.
 
 ## Overlays
 
@@ -79,7 +162,6 @@ until the exit animation ends. High contrast, `forced-colors`, and
 - Keybinding hints show the contributed defaults the consumer passes, not
   user remaps: VS Code exposes no API for extensions to resolve a command's
   effective keybinding.
-- List/selection-row tokens are deferred to the Tree suite (#1037).
 
 ## Codicons
 
@@ -97,4 +179,6 @@ declared CSS exports.
 
 Shared internals are reached through `package.json` subpath imports (`#cx`,
 `#codicons`, `#storybook`). These resolve only inside this package and ship
-with it, so they survive a standalone NPM split.
+with it, so they survive a standalone NPM split. Component families keep
+their own internals (contexts, stores) inside their folder and import them
+relatively, so a family can lift out wholesale.

packages/ui/src/components/Tree/Tree.css

@@ -0,0 +1,171 @@
+.ui-tree {
+	--ui-tree-indent-size: 8px;
+	--ui-tree-row-height: 22px;
+	width: 100%;
+	min-width: 0;
+	outline: 0;
+}
+
+.ui-tree-item {
+	outline: 0;
+}
+
+.ui-tree-item__row {
+	position: relative;
+	display: flex;
+	align-items: center;
+	height: var(--ui-tree-row-height);
+	padding-inline-end: var(--ui-spacing-120);
+	background: var(--ui-tree-row-background, transparent);
+	cursor: pointer;
+	user-select: none;
+}
+
+/* Native skips hover on selected and focused rows, keeping their outlines. */
+.ui-tree-item:not([aria-selected="true"]):not(.ui-tree-item--focused)
+	> .ui-tree-item__row:hover {
+	color: var(--ui-list-hover-foreground);
+	background: var(--ui-list-hover-background);
+	outline: 1px dashed var(--ui-list-hover-outline);
+	outline-offset: -1px;
+}
+
+.ui-tree-item[aria-selected="true"] > .ui-tree-item__row {
+	color: var(--ui-list-inactive-selection-foreground);
+	background: var(--ui-list-inactive-selection-background);
+	outline: 1px dotted var(--ui-list-selection-outline);
+	outline-offset: -1px;
+}
+
+.ui-tree--focused .ui-tree-item[aria-selected="true"] > .ui-tree-item__row {
+	color: var(--ui-list-active-selection-foreground);
+	background: var(--ui-list-active-selection-background);
+}
+
+.ui-tree-item__indent {
+	position: absolute;
+	inset-block: 0;
+	inset-inline-start: calc(2 * var(--ui-tree-indent-size));
+	display: flex;
+	pointer-events: none;
+}
+
+/* The native list's inactive focus outline: kept while the tree is blurred. */
+.ui-tree:not(.ui-tree--focused) .ui-tree-item--focused > .ui-tree-item__row {
+	outline: 1px dotted var(--ui-list-inactive-focus-outline);
+	outline-offset: -1px;
+}
+
+/* One guide per ancestor, like the native tree's .indent-guide. */
+.ui-tree-item__indent-slot {
+	box-sizing: border-box;
+	width: var(--ui-tree-indent-size);
+	flex: none;
+	border-inline-start: 1px solid transparent;
+}
+
+/* Never overlapping selectors, so neither can override the other. */
+.ui-tree-item__indent-slot--active {
+	border-inline-start-color: var(--ui-tree-indent-guide-active);
+}
+
+.ui-tree:hover
+	.ui-tree-item__indent-slot:not(.ui-tree-item__indent-slot--active) {
+	border-inline-start-color: var(--ui-tree-indent-guide-inactive);
+}
+
+.ui-tree-item__chevron {
+	display: flex;
+	align-items: center;
+	justify-content: center;
+	width: 16px;
+	height: var(--ui-tree-row-height);
+	padding-inline-start: calc(var(--ui-tree-level) * var(--ui-tree-indent-size));
+	padding-inline-end: 6px;
+	flex: none;
+	transform: translateX(3px);
+}
+
+.ui-tree-item__chevron:dir(rtl) {
+	transform: translateX(-3px);
+}
+
+/* Keep 3px so leaf icons clear the innermost guide and line up with twisties. */
+.ui-tree--explorer
+	.ui-tree-item:not([aria-expanded])
+	> .ui-tree-item__row
+	> .ui-tree-item__chevron {
+	width: 3px;
+	padding-inline-end: 0;
+	visibility: hidden;
+}
+
+.ui-tree-item__chevron > .ui-icon {
+	width: 10px;
+	font-size: 10px;
+}
+
+.ui-tree-item__content {
+	display: flex;
+	align-items: center;
+	min-width: 0;
+	flex: 1;
+	line-height: var(--ui-tree-row-height);
+	overflow: hidden;
+	white-space: nowrap;
+}
+
+.ui-tree-item__content > .ui-icon {
+	margin-inline-end: var(--ui-spacing-60);
+	flex: none;
+}
+
+.ui-tree-item__action {
+	display: none;
+	align-items: center;
+	align-self: stretch;
+	flex: none;
+	gap: 2px;
+}
+
+.ui-tree-item[aria-selected="true"] > .ui-tree-item__row .ui-tree-item__action,
+.ui-tree-item__row:hover .ui-tree-item__action,
+.ui-tree-item--focused > .ui-tree-item__row .ui-tree-item__action,
+.ui-tree-item__row:focus-within .ui-tree-item__action {
+	display: inline-flex;
+}
+
+@media (prefers-reduced-motion: no-preference) {
+	.ui-tree-item__indent-slot {
+		transition: border-color 100ms linear;
+	}
+}
+
+@media (forced-colors: active) {
+	.ui-tree-item:not([aria-selected="true"]) > .ui-tree-item__row:hover,
+	.ui-tree-item[aria-selected="true"] > .ui-tree-item__row {
+		color: HighlightText;
+		background: Highlight;
+	}
+
+	.ui-tree:hover .ui-tree-item__indent-slot,
+	.ui-tree-item__indent-slot--active {
+		border-color: CanvasText;
+	}
+}
+
+:where(:root:not([data-ui-style="stable"])) .ui-tree-item__row {
+	margin-inline: var(--ui-spacing-40);
+	border-radius: var(--ui-radius-small);
+}
+
+.ui-tree--focused .ui-tree-item--focused > .ui-tree-item__row {
+	outline: 1px solid var(--ui-list-focus-outline);
+	outline-offset: -1px;
+}
+
+.ui-tree--focused
+	.ui-tree-item--focused[aria-selected="true"]
+	> .ui-tree-item__row {
+	outline-color: var(--ui-list-focus-and-selection-outline);
+}