fix: correct split view tab bar positioning, redraw, and focus

[hybridmachine]

Summary

• Fix tab bar coordinates in split mode to use contentView coordinates instead of splitView-local coordinates, preventing misalignment when panels (file browser, function list) are visible
• Force Scintilla redraw via SCI_COLOURISE after unsplit reparenting so the editor content is displayed
• Remove redundant layoutSplitTopTabBars() call that ran before relayoutPanels() with stale frame data
• Add ScintillaBridge_focus() after unsplit to restore keyboard focus to the remaining editor

Test plan

• Split with no panels → both tab bars aligned with their editor panes
• Unsplit → remaining editor shows content and accepts keyboard input
• Split with file browser enabled → tab bars offset correctly (not overlapping panel)
• Split with function list enabled → tab bars positioned correctly
• Split with both panels → correct tab bar alignment
• Unsplit from right pane active → left pane gets focus, shows content
• Resize window while split with panels → tab bars follow correctly
• Drag split divider with panels → tab bars reposition correctly

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Fixes split/unsplit UI correctness on macOS by ensuring tab bars are positioned in the correct coordinate space, and by restoring editor rendering/focus when collapsing back to a single view.

Changes:

• Compute split tab bar X positions using splitView’s offset within contentView to avoid misalignment when side panels are visible.
• Force a Scintilla re-colourise after unsplit reparenting to ensure editor content redraws.
• Remove a pre-relayoutPanels() tab layout call and restore focus to the remaining Scintilla view after unsplitting.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

doUnsplit() now always calls ScintillaBridge_focus(ctx().scintillaView) at the end. If the incremental search bar is visible, this will steal keyboard focus from the search field (showIncrementalSearch explicitly makes the search field first responder, and document switching keeps that focus). Consider only focusing Scintilla when incremental search is not visible, or explicitly restoring focus to the search field when isIncrementalSearchVisible() is true (or when the previous first responder was the closing right-hand Scintilla view).

Suggested change

	ScintillaBridge_focus(ctx().scintillaView);
	if (!isIncrementalSearchVisible())
	ScintillaBridge_focus(ctx().scintillaView);