perf(editor): mount TipTap editors after the navigation paint

[bhackett1024]

Why

Opening an issue stuttered: navigating from the Issues list to the detail page delivered one synchronous 52.6ms commit (~3 dropped frames), 83.8% of it from mounting two TipTap/ProseMirror editors — the description editor and the comment composer — whose useEditor initialization costs ~20ms of synchronous main-thread work per instance. PR #56 moved the editor download off the critical path, but with the chunk cached every issue open still paid the mount cost inside the navigation frame. Report: bug-mupr5tsb-7yuz.

What

Editors now wait for the navigation frame to actually paint (double requestAnimationFrame) before mounting, inside RichTextEditorImpl. While deferred, the shell renders a static preview of the same chrome:

• a toolbar skeleton reserving the real toolbar's exact height — no layout shift when the toolbar appears;
• the description HTML via dangerouslySetInnerHTML — the same trust model as the editor itself and ActivityFeed's comment rendering — or, for an empty document, TipTap's own placeholder paragraph markup (p.is-editor-empty[data-placeholder]) so the placeholder text is present during the swap.

Trade-off: the interactive editor arrives 1–2 frames after the page paints, instead of the page waiting ~3 frames to paint at all. Alternatives rejected: wrapping the navigation state in a transition (does not remove the synchronous editor-effect cost from the commit) and click-to-edit descriptions (changes product behavior).

How to Review

• The change is confined to src/components/ui/RichTextEditorImpl.tsx — deliberately zero overlap with PR perf(bundle): code-split the Tiptap editor stack out of the eager bundle #57's in-flight chunk-splitting refactor (wrapper + consumer files).
• immediatelyRender: false and its destroy-timer rationale (bug-mun3jox6-810l) are preserved; the deferral gates mounting the component, not the editor creation race.
• All three editor consumers (detail description, comment composer, new-issue dialog) get the deferral; none pass className, and the dialog does not autofocus the editor.

Verification

• npm run lint and npm run build (tsc -b + vite) green locally; the editor impl still builds as its own lazy chunk.
• Self Healing QA verification against the exact-head Netlify preview is queued through the operate loop.

Human author: Brian Hackett (bhackett@replay.io)

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