[lexical-playground] Chore: De-flake collab "Undo with collaboration on" e2e test

[etrepum]

Description

The "Undo with collaboration on" test intermittently failed on webkit (and is racy on all browsers) with a partial undo: e.g. after undoing the "hello world again" burst the middle paragraph still showed "hello world aga", or after undoing the checklist a list item or "a" remained, instead of the expected empty paragraph.

Root cause: in collab, @lexical/history is disabled in favor of the Yjs UndoManager, which coalesces consecutive local edits into one undo stack item only while each lands within captureTimeout (500ms) of the previous, measured with Date.now(). Unlike the local-history clock that advanceHistoryClock drives, that timer is not injectable, so under CI load a stall longer than the window silently splits a single logical edit group across multiple stack items. The test issues one undo() per group and asserts the whole group reverts, so a split leaves a partial revert.

Fix: add freezeCollabUndoGrouping(), which sets the UndoManager's captureTimeout to Infinity for the edited frame. Grouping is then driven solely by explicit boundaries -- advanceHistoryClock's stopCapturing() and the stopCapturing() the UndoManager runs automatically after every undo/redo (both reset lastChange to 0). Edits in between always coalesce regardless of timing, so a burst reverts as a unit. This does not change any asserted behavior: a run that happened to stay under the 500ms window already grouped the same way.

Test plan

Test changes only

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Project	Deployment	Actions	Updated (UTC)
lexical	Ready	Preview, Comment	Jun 4, 2026 4:02pm
lexical-playground	Ready	Preview, Comment	Jun 4, 2026 4:02pm

[potatowagon]

Reviewed by Navi (Tater Thoughts Bobblehead) on behalf of @potatowagon.

LGTM ✅ — Excellent de-flaking fix for collab undo tests.

What this does: Introduces a freezeCollabUndoGrouping(page) test utility that sets the Yjs UndoManager's captureTimeout to Infinity during test execution. This ensures that undo grouping is purely boundary-driven (explicit stopCapturing() calls) rather than wall-clock-driven (500ms default window), preventing CI stalls from splitting edit groups across multiple undo stack items.

What I checked:

• ✅ Logic correctness: The fix correctly identifies the root cause — Yjs UndoManager coalesces edits only within its captureTimeout window. Under CI load, a >500ms stall between keystrokes splits what should be a single group. Setting to Infinity removes the time dimension while keeping boundary-driven grouping intact.
• ✅ Edge cases: The helper is properly guarded (if (!IS_COLLAB) return), correctly polls via waitForFunction until the UndoManager is available (handles async collab connection), accesses it via the well-known symbol Symbol.for('@lexical/yjs/UndoManager'), and is idempotent.
• ✅ No behavioral change: The test assertions remain unchanged — a run that naturally stayed under 500ms already grouped the same way. Only flaky intermittent splits are eliminated.
• ✅ Thorough documentation: The JSDoc explains the mechanism, rationale, and interaction with advanceHistoryClock.
• ✅ No www compat concerns — test-only change.
• ✅ CI: All checks green.

Safe to land. This eliminates a well-understood class of CI flakiness without changing any test semantics.