editor: don't lay out the editor part during state restore before it is shown

[sandy081]

What

EditorPart.doApplyGridState called this.doLayout(this._contentDimension) unconditionally while applying an editor working set (state restore). When the editor part has never been laid out — _contentDimension is still undefined — that call throws (Cannot read properties of undefined (reading 'width')) and aborts the method before the onDidAddGroup events are fired for the recreated groups.

As a result, the restored editor groups are never registered with IEditorService (its onDidAddGroup → registerGroupListeners never runs), so their onWillOpenEditor has no listeners.

This guards the layout call so it only runs when the part already has a content dimension. The grid is laid out later when the part is first shown.

Why

In the Agents window, the editor part can stay hidden across a reload (e.g. the user closed the whole side pane). On reload the per-session layout controller restores the session's editor working set into that still-hidden, never-laid-out editor part, hitting the crash above. Because the restored group was never registered with the editor service, the Agents window's onWillOpenEditor-driven editor-part reveal never fired — so clicking Changes opened the editor into an orphaned group without ever revealing the editor part.

Repro (Agents window):

1. Open a session, click Changes (editor + aux bar visible), open the sessions list (all four parts visible).
2. Toggle the side pane closed (hides editor + aux bar).
3. Reload the window.
4. Click Changes → editor part did not open.

Notes for reviewers

• The fix mirrors the existing defensive !this._contentDimension guard already used elsewhere in editorPart.ts (e.g. updateTopRightGroupContextKey).
• Low risk for the main workbench: the main editor part is always laid out at startup, so _contentDimension is defined there and behavior is unchanged. This only affects the case where state is applied into a part that has never been shown.
• Added a regression unit test in editorGroupsService.test.ts that applies state into a never-laid-out part and asserts it does not throw and still fires onDidAddGroup for the restored groups (verified it fails with the original TypeError without the fix).

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[vs-code-engineering]

📬 CODENOTIFY

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

@benibenj

Matched files:

• src/vs/workbench/browser/parts/editor/editorPart.ts
• src/vs/workbench/services/editor/test/browser/editorGroupsService.test.ts

[Copilot]

Pull request overview

Fixes an editor working-set restore crash by avoiding an early layout pass when the editor part has never been laid out (hidden/not-yet-shown), ensuring restored groups still emit onDidAddGroup and get properly registered (notably impacting the Agents window restore flow).

Changes:

• Guard EditorPart.doApplyGridState layout so it only runs when _contentDimension is available.
• Add a regression unit test that applies state into a never-laid-out editor part and asserts restore completes and group-added events fire.

Show a summary per file

File	Description
src/vs/workbench/browser/parts/editor/editorPart.ts	Avoids calling doLayout during state restore when _contentDimension is still undefined, preventing a restore-time crash and ensuring group events still fire.
src/vs/workbench/services/editor/test/browser/editorGroupsService.test.ts	Adds a regression test covering state restore into an editor part that has never been laid out, validating no throw and that restored groups are registered via onDidAddGroup.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Low