sessions: restore minimized split on activation

Expand a minimized Sessions or Editor part when it receives pointer or keyboard focus while preserving the widths of unrelated panes.\n\nCo-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sandy081

.github/skills/sessions/SKILL.md

@@ -26,6 +26,8 @@ Then read the relevant spec for the area you are changing (see table below). If
 
 ## Common Pitfalls
 
+- **Minimum-size activation across the Sessions/Editor split must be symmetric**: when either part is at minimum width, pointer or keyboard activation expands it by shrinking its sibling to minimum width. Implement and test both directions together; a one-sided Sessions-only handler does not match editor-grid behavior.
+
 - **Animation performance must preserve perceptual smoothness**: reducing a continuous title shimmer to 10 stepped updates per second makes the sweep visibly choppy even if paint counts improve. Use a smooth baseline such as 30 updates per second, then measure the remaining performance win; do not optimize decorative motion by callback counts alone.
 
 - **Pet placement must align the visible sprite, not only its absolute-positioning box**: anchoring the button at `bottom: 100%` leaves the pet visually detached because the input stack has top padding and transient confirmation/question surfaces add their own top margin. Keep the host on the complete stack and derive the optical offset from the actual input-to-host inset, capped at the confirmation/question alignment; one fixed deeper offset makes the bare input look overlapped.

src/vs/sessions/LAYOUT.md

@@ -64,6 +64,8 @@ The **Sessions Part is the flexible ("remaining width") view** in the top-right
 
 The Sessions Part-to-Editor gap and the gap above the bottom Panel share `AGENTS_FLOATING_PANEL_GAP` in TypeScript layout and its registered CSS token, `--vscode-agents-layout-floatingPanelGap`. Their grid sashes keep the split boundaries unchanged, but expand and shift their hit areas to fill those visual gaps exactly. Each shows the standard persistent three-dot gripper at rest and yields to the full sash highlight while hovered or dragged. The Auxiliary Bar's leading padding and part-internal sashes retain their independent geometry.
 
+When either the Sessions Part or Editor has been resized to its minimum width, activating that part by pointer or keyboard focus restores it to the available width by resizing its sibling to minimum width. This mirrors minimized editor-group activation while targeting only the Sessions/Editor pair, so the Sidebar and Auxiliary Bar retain their established widths.
+
 Editor-content overlays must use the editor pane container rather than the editor-group root. In the single-pane layout, the group spans both the editor and the docked detail panel while the pane container is inset to the editor's actual bounds; anchoring feedback controls such as the Submit toolbar to the group would place them over the detail panel.
 
 ### 2.3 Layout Priority Model

src/vs/sessions/browser/workbench.ts

@@ -9,7 +9,7 @@ import './media/workbench.css';
 import './media/phoneLayout.css';
 import { Disposable, DisposableStore, IDisposable, toDisposable } from '../../base/common/lifecycle.js';
 import { Emitter, Event, setGlobalLeakWarningThreshold } from '../../base/common/event.js';
-import { addDisposableListener, getActiveDocument, getActiveElement, getClientArea, getWindowId, getWindows, IDimension, isAncestorUsingFlowTo, isHTMLElement, size, Dimension, runWhenWindowIdle } from '../../base/browser/dom.js';
+import { addDisposableGenericMouseDownListener, addDisposableListener, EventType, getActiveDocument, getActiveElement, getClientArea, getWindowId, getWindows, IDimension, isAncestorUsingFlowTo, isHTMLElement, size, Dimension, runWhenWindowIdle } from '../../base/browser/dom.js';
 import { DeferredPromise, RunOnceScheduler } from '../../base/common/async.js';
 import { isFullscreen, onDidChangeFullscreen, isChrome, isFirefox, isSafari } from '../../base/browser/browser.js';
 import { mark } from '../../base/common/performance.js';
@@ -1089,6 +1089,8 @@ export class Workbench extends Disposable implements IAgentWorkbenchLayoutServic
 		editorPartContainer.classList.add('part', 'editor');
 		editorPartContainer.id = Parts.EDITOR_PART;
 		editorPartContainer.setAttribute('role', 'main');
+		this._register(addDisposableListener(editorPartContainer, EventType.FOCUS_IN, () => this._restoreEditorPartOnActivation()));
+		this._register(addDisposableGenericMouseDownListener(editorPartContainer, () => this._restoreEditorPartOnActivation()));
 		this._editorPartContainer = editorPartContainer;
 
 		mark('code/willCreatePart/workbench.parts.editor');
@@ -1103,6 +1105,8 @@ export class Workbench extends Disposable implements IAgentWorkbenchLayoutServic
 		sessionsPartContainer.classList.add('part', 'sessionspart', 'basepanel', 'right', AGENTS_PART_CARD_CLASS);
 		sessionsPartContainer.id = Parts.SESSIONS_PART;
 		sessionsPartContainer.setAttribute('role', 'main');
+		this._register(addDisposableListener(sessionsPartContainer, EventType.FOCUS_IN, () => this._restoreSessionsPartOnActivation()));
+		this._register(addDisposableGenericMouseDownListener(sessionsPartContainer, () => this._restoreSessionsPartOnActivation()));
 
 		mark(`code/willCreatePart/${Parts.SESSIONS_PART}`);
 		this.getPart(Parts.SESSIONS_PART).create(sessionsPartContainer);
@@ -1111,6 +1115,34 @@ export class Workbench extends Disposable implements IAgentWorkbenchLayoutServic
 		this.mainContainer.appendChild(sessionsPartContainer);
 	}
 
+	private _restoreSessionsPartOnActivation(): void {
+		if (!this.workbenchGrid || !this.isVisible(Parts.EDITOR_PART, mainWindow)) {
+			return;
+		}
+
+		this._restoreMinimizedPartOnActivation(this.sessionsPartView, this.editorPartView);
+	}
+
+	private _restoreEditorPartOnActivation(): void {
+		if (!this.workbenchGrid || !this.isVisible(Parts.EDITOR_PART, mainWindow) || !this.isVisible(Parts.SESSIONS_PART)) {
+			return;
+		}
+
+		this._restoreMinimizedPartOnActivation(this.editorPartView, this.sessionsPartView);
+	}
+
+	private _restoreMinimizedPartOnActivation(target: ISerializableView, sibling: ISerializableView): void {
+		const targetSize = this.workbenchGrid.getViewSize(target);
+		if (targetSize.width !== target.minimumWidth) {
+			return;
+		}
+
+		const siblingSize = this.workbenchGrid.getViewSize(sibling);
+		if (siblingSize.width > sibling.minimumWidth) {
+			this.workbenchGrid.resizeView(sibling, { width: sibling.minimumWidth, height: siblingSize.height });
+		}
+	}
+
 	private createCustomViewGridPart(): void {
 		const customViewGridPartContainer = document.createElement('div');
 		customViewGridPartContainer.classList.add('part', 'customviewgridpart', 'basepanel', 'right', AGENTS_PART_CARD_CLASS);

src/vs/sessions/test/browser/workbench.test.ts

@@ -60,6 +60,8 @@ suite('Sessions - Workbench', () => {
 	const updateMobileCustomViewNavigation = Reflect.get(Workbench.prototype, '_updateMobileCustomViewNavigation') as (this: ITestWorkbench) => void;
 	const isVisible = Workbench.prototype.isVisible as (this: ITestWorkbench, part: Parts) => boolean;
 	const toggleSecondarySideBar = Workbench.prototype.toggleSecondarySideBar as (this: ITestWorkbench) => void;
+	const restoreSessionsPartOnActivation = Reflect.get(Workbench.prototype, '_restoreSessionsPartOnActivation') as (this: ITestWorkbench) => void;
+	const restoreEditorPartOnActivation = Reflect.get(Workbench.prototype, '_restoreEditorPartOnActivation') as (this: ITestWorkbench) => void;
 
 	// --- Harness ------------------------------------------------------------
 
@@ -147,8 +149,8 @@ suite('Sessions - Workbench', () => {
 	}
 
 	function createHost(options: IHostOptions = {}): ITestWorkbench {
-		const editorPartView = {};
-		const sessionsPartView = {};
+		const editorPartView = { minimumWidth: 300 };
+		const sessionsPartView = { minimumWidth: 300 };
 		const sideBarPartView = {};
 		const auxiliaryBarPartView = {};
 		const panelPartView = {};
@@ -288,6 +290,31 @@ suite('Sessions - Workbench', () => {
 
 	// --- Editor split / reveal ---------------------------------------------
 
+	test('activating a minimized Sessions or Editor Part resizes its sibling to minimum width', () => {
+		const sessionsMinimized = createHost({ sessionsWidth: 300, editorWidth: 700, partVisibility: { editor: true } });
+		const editorMinimized = createHost({ sessionsWidth: 700, editorWidth: 300, partVisibility: { editor: true } });
+		const neitherMinimized = createHost({ sessionsWidth: 301, editorWidth: 301, partVisibility: { editor: true } });
+		const editorHidden = createHost({ sessionsWidth: 300, editorWidth: 700, partVisibility: { editor: false } });
+
+		restoreSessionsPartOnActivation.call(sessionsMinimized);
+		restoreEditorPartOnActivation.call(editorMinimized);
+		restoreSessionsPartOnActivation.call(neitherMinimized);
+		restoreEditorPartOnActivation.call(neitherMinimized);
+		restoreSessionsPartOnActivation.call(editorHidden);
+
+		assert.deepStrictEqual([
+			sessionsMinimized.resizes,
+			editorMinimized.resizes,
+			neitherMinimized.resizes,
+			editorHidden.resizes,
+		], [
+			[{ width: 300, height: 800 }],
+			[{ width: 300, height: 800 }],
+			[],
+			[],
+		]);
+	});
+
 	test('tracks editor pane visibility across editor and auxiliary bar changes', () => {
 		const host = createHost({ partVisibility: { editor: false, auxiliaryBar: true } });