fix(editor): prevent focus loss when typing in header/footer editors (SD-1993)

[tupizz]

Summary

Fixes SD-1993: headers and footers lose focus after the first keystroke, making them effectively uneditable.

Root cause

Every keystroke in a header/footer PM editor triggers a full body rerender cycle:

User types in H/F editor
  → contentChanged event → requestRerender()
  → RAF → #rerender() → painter.paint()
  → renderVirtualized() → updateVirtualWindow()
  → appendChild(pageElement)   ← browser fires blur on focused H/F editor

DomPainter.updateVirtualWindow() has an ordering loop that re-appends all mounted page elements via appendChild. Calling appendChild on an element already in the DOM is a move-to-end operation. The browser fires a blur event on any focused element inside the moved subtree. This kills focus on the header/footer PM editor after every keystroke.

All three reported symptoms stem from this single focus loss:

• Header only accepts first keystroke: first char triggers rerender → blur → second char has no target
• Space scrolls to bottom: focus loss + spacer height recalculation → browser adjusts scroll
• Footer text appears in body: after focus loss, input bridge may route keystrokes to the body editor

Fix

1. Cursor-based DOM reconciliation in updateVirtualWindow (renderer.ts)

Replace the unconditional appendChild ordering loop with cursor-based reconciliation. Tracks a cursor pointer walking through the existing children. For each mounted page, if it's already at the cursor position, skip the DOM mutation and advance. Only call insertBefore when elements are actually out of order.

In the common case (H/F typing with no scroll or window change), pages are already in the correct order after clearGapSpacers() removes old gaps. Zero moves = zero blur = focus preserved.

2. Focus save/restore safety net (PresentationEditor.ts)

Captures the active H/F editor's focus state before #rerender(). In the finally block, if focus was lost (e.g. from edge cases like scroll during editing, viewport resize, page count changes), restores it via activeHfEditor.view.focus().

Test plan

• All 645 painter-dom tests pass
• All 7411 super-editor tests pass
• Pre-commit hooks pass (format, lint, typecheck, commitlint)
• Manual: load document with header/footer, double-click header, type multiple characters rapidly. All characters appear, no scroll jump.
• Manual: double-click footer, type text. Text appears in footer only, not in body.
• Manual: press Escape to exit, verify changes persist in static render.
• Manual: verify normal body editing and page virtualization (scrolling) still work correctly.

[linear]

SD-1993 BUG: Headers/Footers Do Not Support Typing

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 41a9dec4be

ℹ️ About Codex in GitHub

Codex has been enabled to automatically review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

When you sign up for Codex through ChatGPT, Codex can also answer questions or update the PR, like "@codex address that feedback".

[chatgpt-codex-connector]

Gate focus recovery on current H/F session

The new focus recovery in #flushRerenderQueue always refocuses the previously-captured header/footer editor when it lost focus during rerender, but it does not verify that this editor is still the active session. If the user intentionally exits header/footer mode (or switches to another header/footer editor) while #rerender() is awaiting, exitMode() can correctly focus the body editor and then this block will immediately steal focus back to the stale editor, causing unexpected focus jumps and mode-switch glitches.

Useful? React with 👍 / 👎.