Issue 4527: AI menu streaming anchor can be undefined

Tracker

• Source type: GitHub issue
• Source id: #4527
• Title: AIMenu: Cannot read properties of undefined (reading '0') during streaming
• URL: https://github.com/udecode/plate/issues/4527
• Task type: bug
• Expected outcome: confirm whether current main can still dereference an undefined AI chat anchor during streaming; if yes, fix it and verify the AI menu remains stable during streaming startup
• Browser surface: yes, but the first reproduction seam may be a targeted React/UI test around AIMenu

Relevant area

• apps/www/src/registry/ui/ai-menu.tsx
• packages/ai/src/react/ai-chat/AIChatPlugin.ts
• packages/ai/src/react/ai-chat/streaming/streamInsertChunk.ts
• any AI menu or AI streaming tests in apps/www or packages/ai

Learnings

• The issue was reported against Plate 49.0.2, so it may be stale
• Current main still contains the exact anchor![0] dereference inside the streaming effect in AIMenu
• No PR referencing #4527 is currently open or merged by title/body search

Plan

1. Inspect the current AI menu and AI chat anchor lifecycle on main.
2. Reproduce the undefined-anchor path on current code with the smallest honest seam.
3. If the bug is live, add a regression test and implement the minimal fix.
4. Run targeted verification, then broader required checks if code changed.
5. Create/update the PR and issue comment only if the bug is still valid and fixed.

Progress

• Fetched the issue and comments.
• Read the task, planning-with-files, learnings-researcher, and bug-reproduction-validator skill docs.
• Searched local solution docs and current code for AI streaming, anchors, and AIMenu.
• Confirmed the current AIMenu implementation still uses const anchorDom = editor.api.toDOMNode(anchor![0])! inside a setTimeout.
• Added a regression test in apps/www/src/registry/ui/ai-menu.spec.tsx that reproduced the current undefined-anchor crash on main.
• Updated apps/www/src/registry/ui/ai-menu.tsx to resolve the anchor inside the timeout, guard missing anchor entries, and clear the timeout on cleanup.
• Verified with targeted tests, pnpm --filter www build, pnpm --filter www typecheck, pnpm lint:fix, and full pnpm check.
• Re-opened the live /blocks/editor-ai surface in the real browser and captured proof at .claude/tmp/4527-ai-menu-browser-proof.png.

Errors

• planning-with-files session catchup script path from the generated skill is missing locally, so catchup could not run in this repo.
• browser-use screenshot writes only accept repo-relative paths, not absolute paths.