sessions: disable unsupported editor splits

src/vs/sessions/LAYOUT.md

@@ -96,6 +96,12 @@ The Editor and Auxiliary Bar compose one side pane next to the active session.
 Editor tabs choose either editor content or a details view while the layout
 coordinators preserve one coherent visibility model.
 
+The main Editor supports exactly one editor group. Its shared multiple-group
+capability is disabled, which removes editor split/grid commands, keybindings,
+menus, and split drop targets; the part also rejects group creation and
+multi-group layout requests from open-to-side and programmatic paths. The
+independent chat grid remains supported.
+
 The durable state and transition catalog lives in
 [SINGLE_PANE_SCENARIOS.md](SINGLE_PANE_SCENARIOS.md). Implementation behavior is
 covered by the layout-controller and single-pane strategy tests.

src/vs/sessions/SINGLE_PANE_SCENARIOS.md

@@ -19,6 +19,10 @@ bar spanning the editor content and a docked detail panel).
   single-pane layout described here. Phone viewports always use the classic
   layout. When the setting is **OFF**, all Agents windows use the classic layout
   and nothing else in this document applies.
+- The main Editor supports exactly one editor group. Editor split/grid commands,
+  keybindings, menus, open-to-side requests, and split drop targets are disabled;
+  programmatic group creation and multi-group layout requests are rejected. This
+  restriction does not apply to the separate chat grid.
 - Companion specs: [Editor presentation](LAYOUT.md#editor-presentation),
   [LAYOUT_CONTROLLER.md](LAYOUT_CONTROLLER.md), and
   [contrib/layout/browser/desktopSessionLayoutController.md](contrib/layout/browser/desktopSessionLayoutController.md).

src/vs/sessions/browser/parts/editorParts.ts

@@ -5,8 +5,10 @@
 
 import './media/editorPart.css';
 import { InstantiationType, registerSingleton } from '../../../platform/instantiation/common/extensions.js';
+import { IEditorGroupView } from '../../../workbench/browser/parts/editor/editor.js';
 import { EditorParts as EditorPartsBase } from '../../../workbench/browser/parts/editor/editorParts.js';
-import { IEditorGroupsService } from '../../../workbench/services/editor/common/editorGroupsService.js';
+import { GroupIdentifier } from '../../../workbench/common/editor.js';
+import { GroupDirection, IEditorGroupsService } from '../../../workbench/services/editor/common/editorGroupsService.js';
 import { IAgentWorkbenchLayoutService } from '../workbench.js';
 import { MainEditorPart } from './editorPart.js';
 import { SinglePaneMainEditorPart } from './singlePaneEditorPart.js';
@@ -21,6 +23,35 @@ export class EditorParts extends EditorPartsBase {
 
 		return editorPart;
 	}
+
+	override moveGroup(group: IEditorGroupView | GroupIdentifier, location: IEditorGroupView | GroupIdentifier, direction: GroupDirection): IEditorGroupView {
+		if (this.involvesSinglePaneMainPart(group, location)) {
+			return this.resolveGroup(group);
+		}
+
+		return super.moveGroup(group, location, direction);
+	}
+
+	override copyGroup(group: IEditorGroupView | GroupIdentifier, location: IEditorGroupView | GroupIdentifier, direction: GroupDirection): IEditorGroupView {
+		if (this.involvesSinglePaneMainPart(group, location)) {
+			return this.resolveGroup(group);
+		}
+
+		return super.copyGroup(group, location, direction);
+	}
+
+	private involvesSinglePaneMainPart(group: IEditorGroupView | GroupIdentifier, location: IEditorGroupView | GroupIdentifier): boolean {
+		return this.mainPart instanceof SinglePaneMainEditorPart
+			&& (this.getPart(group) === this.mainPart || this.getPart(location) === this.mainPart);
+	}
+
+	private resolveGroup(group: IEditorGroupView | GroupIdentifier): IEditorGroupView {
+		const resolvedGroup = typeof group === 'number' ? this.getGroup(group) : group;
+		if (!resolvedGroup) {
+			throw new Error('Invalid editor group provided!');
+		}
+		return resolvedGroup;
+	}
 }
 
 registerSingleton(IEditorGroupsService, EditorParts, InstantiationType.Eager);

src/vs/sessions/browser/parts/singlePaneEditorPart.ts

@@ -4,14 +4,17 @@
  *--------------------------------------------------------------------------------------------*/
 
 import { mainWindow } from '../../../base/browser/window.js';
-import { DisposableMap, MutableDisposable } from '../../../base/common/lifecycle.js';
+import { DisposableMap, IDisposable, MutableDisposable } from '../../../base/common/lifecycle.js';
 import { IConfigurationService } from '../../../platform/configuration/common/configuration.js';
 import { IContextKeyService } from '../../../platform/contextkey/common/contextkey.js';
 import { IInstantiationService } from '../../../platform/instantiation/common/instantiation.js';
 import { IStorageService } from '../../../platform/storage/common/storage.js';
 import { IThemeService } from '../../../platform/theme/common/themeService.js';
-import { IEditorGroupViewOptions, IEditorPartCreationOptions, IEditorPartsView } from '../../../workbench/browser/parts/editor/editor.js';
+import { IEditorGroupView, IEditorGroupViewOptions, IEditorPartCreationOptions, IEditorPartsView } from '../../../workbench/browser/parts/editor/editor.js';
+import { IEditorPartUIState } from '../../../workbench/browser/parts/editor/editorPart.js';
 import { EditorGroupView } from '../../../workbench/browser/parts/editor/editorGroupView.js';
+import { GroupIdentifier } from '../../../workbench/common/editor.js';
+import { EditorGroupLayout, GroupDirection, GroupLayoutArgument, IEditorDropTargetDelegate } from '../../../workbench/services/editor/common/editorGroupsService.js';
 import { Parts } from '../../../workbench/services/layout/browser/layoutService.js';
 import { IHostService } from '../../../workbench/services/host/browser/host.js';
 import { DockedAuxiliaryBarController } from '../dockedAuxiliaryBarController.js';
@@ -154,6 +157,32 @@ export class SinglePaneMainEditorPart extends MainEditorPart {
 		return container;
 	}
 
+	override addGroup(location: IEditorGroupView | GroupIdentifier, _direction: GroupDirection, _groupToCopy?: IEditorGroupView): IEditorGroupView {
+		return this.assertGroupView(location);
+	}
+
+	override applyLayout(layout: EditorGroupLayout): void {
+		if (countEditorGroups(layout.groups) > 1) {
+			return;
+		}
+		super.applyLayout(layout);
+	}
+
+	override createEditorDropTarget(container: unknown, delegate: IEditorDropTargetDelegate): IDisposable {
+		return super.createEditorDropTarget(container, { ...delegate, supportsSplitting: false });
+	}
+
+	override async applyState(state: IEditorPartUIState | 'empty', options?: IEditorGroupViewOptions): Promise<void> {
+		await super.applyState(state, options);
+		this._ensureSingleEditorGroup();
+	}
+
+	private _ensureSingleEditorGroup(): void {
+		if (this.count > 1) {
+			this.mergeAllGroups(this.activeGroup);
+		}
+	}
+
 	/**
 	 * Keeps the docked auxiliary bar aligned after group-local relayouts.
 	 */
@@ -186,3 +215,11 @@ export class SinglePaneMainEditorPart extends MainEditorPart {
 		this._dockedAuxBar?.layout();
 	}
 }
+
+function countEditorGroups(groups: GroupLayoutArgument[]): number {
+	let count = 0;
+	for (const group of groups) {
+		count += group.groups ? countEditorGroups(group.groups) : 1;
+	}
+	return count;
+}

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

@@ -15,8 +15,10 @@ import { DockedAuxiliaryBarController, IDockedAuxiliaryBarHost } from '../../bro
 import { ISidePaneToggleEvent, Workbench } from '../../browser/workbench.js';
 import { DockedEditorSizeMemento, SinglePaneWorkbench } from '../../browser/singlePaneWorkbench.js';
 import { SinglePaneMainEditorPart } from '../../browser/parts/singlePaneEditorPart.js';
+import { EditorParts } from '../../browser/parts/editorParts.js';
 import { DockedEditorInput } from '../../common/dockedEditorInput.js';
 import { EditorInputCapabilities } from '../../../workbench/common/editor.js';
+import { GroupDirection, GroupOrientation } from '../../../workbench/services/editor/common/editorGroupsService.js';
 import { SESSIONS_LIST_MINIMUM_WIDTH } from '../../browser/parts/sidebarPart.js';
 import { Menus } from '../../browser/menus.js';
 import { DEFAULT_NOTIFICATION_ROW_HEIGHT, onDidChangeNotificationRowHeight, setNotificationRowHeight } from '../../../workbench/browser/parts/notifications/notificationsViewer.js';
@@ -1633,6 +1635,81 @@ suite('Sessions - Workbench', () => {
 		});
 	});
 
+	test('single-pane editor part rejects editor group creation and multi-group layouts', () => {
+		const group = {};
+		const addGroup = Reflect.get(SinglePaneMainEditorPart.prototype, 'addGroup') as (location: object, direction: GroupDirection) => object;
+		const applyLayout = Reflect.get(SinglePaneMainEditorPart.prototype, 'applyLayout') as (layout: { orientation: GroupOrientation; groups: object[] }) => void;
+
+		assert.deepStrictEqual({
+			addGroupResult: addGroup.call({ assertGroupView: () => group }, group, GroupDirection.RIGHT),
+			multiGroupLayoutRejected: (() => {
+				applyLayout.call({}, { orientation: GroupOrientation.HORIZONTAL, groups: [{}, {}] });
+				return true;
+			})(),
+		}, {
+			addGroupResult: group,
+			multiGroupLayoutRejected: true,
+		});
+	});
+
+	test('single-pane editor parts reject cross-part group moves and copies', () => {
+		const mainPart = Object.create(SinglePaneMainEditorPart.prototype) as SinglePaneMainEditorPart;
+		const auxiliaryPart = {};
+		const mainGroup = {};
+		const auxiliaryGroup = {};
+		const involvesSinglePaneMainPart = Reflect.get(EditorParts.prototype, 'involvesSinglePaneMainPart') as (group: object, location: object) => boolean;
+		const host = {
+			mainPart,
+			getPart: (group: object) => group === mainGroup ? mainPart : auxiliaryPart,
+			resolveGroup: (group: object) => group,
+			involvesSinglePaneMainPart,
+		};
+		const moveGroup = Reflect.get(EditorParts.prototype, 'moveGroup') as (group: object, location: object, direction: GroupDirection) => object;
+		const copyGroup = Reflect.get(EditorParts.prototype, 'copyGroup') as (group: object, location: object, direction: GroupDirection) => object;
+
+		assert.deepStrictEqual({
+			moveFromMain: moveGroup.call(host, mainGroup, auxiliaryGroup, GroupDirection.RIGHT),
+			moveToMain: moveGroup.call(host, auxiliaryGroup, mainGroup, GroupDirection.RIGHT),
+			copyFromMain: copyGroup.call(host, mainGroup, auxiliaryGroup, GroupDirection.RIGHT),
+			copyToMain: copyGroup.call(host, auxiliaryGroup, mainGroup, GroupDirection.RIGHT),
+		}, {
+			moveFromMain: mainGroup,
+			moveToMain: auxiliaryGroup,
+			copyFromMain: mainGroup,
+			copyToMain: auxiliaryGroup,
+		});
+	});
+
+	test('single-pane editor retains restored editors when collapsing restored groups', () => {
+		const firstEditor = { id: 'first' };
+		const secondEditor = { id: 'second' };
+		const activeGroup = { editors: [secondEditor], activeEditor: secondEditor };
+		const sourceGroup = { editors: [firstEditor], activeEditor: firstEditor };
+		const host = {
+			count: 2,
+			activeGroup,
+			groups: [sourceGroup, activeGroup],
+			mergeAllGroups(target: typeof activeGroup) {
+				target.editors.unshift(...sourceGroup.editors);
+				this.groups = [target];
+				this.count = 1;
+			},
+		};
+		const ensureSingleEditorGroup = Reflect.get(SinglePaneMainEditorPart.prototype, '_ensureSingleEditorGroup') as () => void;
+
+		ensureSingleEditorGroup.call(host);
+
+		assert.deepStrictEqual({
+			groupCount: host.count,
+			editors: host.activeGroup.editors.map(editor => editor.id),
+			activeEditor: host.activeGroup.activeEditor.id,
+		}, {
+			groupCount: 1,
+			editors: ['first', 'second'],
+			activeEditor: 'second',
+		});
+	});
+
 	test('applies an even split when revealing the docked editor with no captured width even after the initial split', () => {
 		const host = createHost({ single: true, sessionsWidth: 1000, windowWidth: 1300, hasAppliedInitialEditorSplit: true, dockedWidth: 300, editorWidth: 300, partVisibility: { editor: false, auxiliaryBar: true } });
 

src/vs/workbench/browser/parts/editor/editorDropTarget.ts

@@ -63,6 +63,7 @@ class DropOverlay extends Themable {
 
 	constructor(
 		private readonly groupView: IEditorGroupView,
+		private readonly supportsSplitting: boolean,
 		@IThemeService themeService: IThemeService,
 		@IConfigurationService private readonly configurationService: IConfigurationService,
 		@IInstantiationService private readonly instantiationService: IInstantiationService,
@@ -181,8 +182,8 @@ class DropOverlay extends Themable {
 				// Position overlay and conditionally enable or disable
 				// editor group splitting support based on setting and
 				// keymodifiers used.
-				let splitOnDragAndDrop = !!this.groupView.groupsView.partOptions.splitOnDragAndDrop;
-				if (this.isToggleSplitOperation(e)) {
+				let splitOnDragAndDrop = this.supportsSplitting && !!this.groupView.groupsView.partOptions.splitOnDragAndDrop;
+				if (this.supportsSplitting && this.isToggleSplitOperation(e)) {
 					splitOnDragAndDrop = !splitOnDragAndDrop;
 				}
 				this.positionOverlay(e.offsetX, e.offsetY, isDraggingGroup, splitOnDragAndDrop);
@@ -388,6 +389,16 @@ class DropOverlay extends Themable {
 		const editorControlWidth = this.groupView.element.clientWidth;
 		const editorControlHeight = this.groupView.element.clientHeight - this.getOverlayOffsetHeight();
 
+		if (!enableSplitting) {
+			this.doPositionOverlay({ top: '0', left: '0', width: '100%', height: '100%' });
+			this.toggleDropIntoPrompt(true);
+			const overlay = assertReturnsDefined(this.overlay);
+			overlay.style.opacity = '1';
+			setTimeout(() => overlay.classList.add('overlay-move-transition'), 0);
+			this.currentDropOperation = { splitDirection: undefined };
+			return;
+		}
+
 		let edgeWidthThresholdFactor: number;
 		let edgeHeightThresholdFactor: number;
 		if (enableSplitting) {
@@ -644,7 +655,7 @@ export class EditorDropTarget extends Themable {
 			if (!this.overlay) {
 				const targetGroupView = this.findTargetGroupView(target);
 				if (targetGroupView) {
-					this._overlay = this.instantiationService.createInstance(DropOverlay, targetGroupView);
+					this._overlay = this.instantiationService.createInstance(DropOverlay, targetGroupView, this.delegate.supportsSplitting !== false);
 				}
 			}
 		}

src/vs/workbench/services/editor/common/editorGroupsService.ts

@@ -203,6 +203,11 @@ export interface IEditorDropTargetDelegate {
 	 * A helper to figure out if the drop target contains the provided group.
 	 */
 	containsGroup?(groupView: IEditorGroup): boolean;
+
+	/**
+	 * Whether the drop target supports creating editor groups.
+	 */
+	readonly supportsSplitting?: boolean;
 }
 
 /**