sessions: disable unsupported editor splits

[Sandeep Somavarapu (sandy081)]

Fixes #331470

Summary

• add an editor-part capability that disables unsupported multi-group interactions only in the single-pane Agents Window main editor
• hide and reject split/grid commands, keybindings, menus, drag-and-drop targets, open-to-side requests, restored layouts, and cross-part group moves/copies
• preserve classic and normal editor layouts, auxiliary editor windows, and the separate multi-chat grid
• document the single-group contract and add targeted regressions

Validation

• npm run compile
• npm run hygiene
• npm run typecheck-client
• npm run valid-layers-check
• targeted Sessions unit tests (127 passing)
• targeted ESLint

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

Benjamin Christopher Simmonds (@benibenj)

Matched files:

• src/vs/sessions/browser/parts/editorParts.ts
• src/vs/sessions/browser/parts/singlePaneEditorPart.ts
• src/vs/workbench/browser/parts/editor/editorDropTarget.ts
• src/vs/workbench/services/editor/common/editorGroupsService.ts

[Copilot]

Pull request overview

Enforces the single-group contract for the Agents Window’s single-pane main editor.

Changes:

• Adds capability-based command, menu, and drag/drop restrictions.
• Rejects group creation, multi-group layouts, and cross-part operations.
• Adds documentation and regression tests.

Show a summary per file

File	Description
src/vs/workbench/common/contextkeys.ts	Defines the capability context key.
src/vs/workbench/browser/parts/editor/editorPart.ts	Publishes per-part capability state.
src/vs/workbench/browser/parts/editor/editorDropTarget.ts	Restricts split drop targets.
src/vs/workbench/browser/parts/editor/editorActions.ts	Gates multi-group actions.
src/vs/workbench/browser/parts/editor/editor.contribution.ts	Hides unsupported menu items.
src/vs/workbench/browser/contextkeys.ts	Initializes the global context key.
src/vs/sessions/browser/workbench.ts	Configures single-pane capability state.
src/vs/sessions/browser/parts/singlePaneEditorPart.ts	Enforces one main editor group.
src/vs/sessions/browser/parts/editorParts.ts	Restricts cross-part group operations.
src/vs/sessions/test/browser/workbench.test.ts	Tests group restrictions.
src/vs/sessions/contrib/editor/test/browser/editor.contribution.test.ts	Tests action and menu gating.
src/vs/sessions/LAYOUT.md	Documents the editor invariant.
src/vs/sessions/SINGLE_PANE_SCENARIOS.md	Records supported single-pane behavior.

Review details

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• Files reviewed: 13/13 changed files
• Comments generated: 3
• Review effort level: Balanced

[Mohammad javad Dianat (dianatofficial)]

Great catch on this edge case. Fallback logic is robust.