feat(xaml-designer): bidirectional canvas↔code selection sync — untested, needs QA

[abbaye]

Feature

Bidirectional selection sync between the XAML Designer canvas and the code editor (VS-like).

Commits: 73bca10e, 6e553bc9 on branch DebugNewFeatures

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What was implemented

Canvas → Code

• Clicking an element on the canvas navigates the code editor caret to the element's opening tag line.
• Uses XamlSourceLocationService.FindElementStartLine (XDocument + IXmlLineInfo).

Code → Canvas

• Moving the caret in the code editor (no active text selection) selects the matching element on the canvas.
• Debounced at 150ms via _codeToCanvasSyncTimer; fires on both CaretMoved (line changes) and SelectionChanged (same-line clicks).
• Uses XamlSourceLocationService.FindUidAtLine (XmlReader + IXmlLineInfo, pre-order UID traversal).

Quality fixes (commit 6e553bc9)

• Post-render preserve selection: canvas no longer loses selection after every 500ms debounce re-render.
• Silent selection API: SelectElement(suppressEvent:true) / SelectElementByUid(suppressEvent:true) for programmatic restores.
• ApplyXamlToCode guard: _isSyncingSelection=true prevents LoadFromString caret-reset from triggering spurious root-element sync.
• Caret theft prevention: skip NavigateTo when code editor is already on the correct line.

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Test scenarios to validate

• Click Button on canvas → code editor caret jumps to <Button line
• Press Escape on canvas (select parent) → code editor moves to parent's line
• Click on <Button in code editor → Button gets selection adorner on canvas
• Click same line multiple times → canvas still re-syncs correctly
• Edit Width="100" → canvas re-renders → element stays selected (not deselected)
• Drag resize handle, release → code updates → canvas re-renders → element stays selected, code caret not stolen
• Click Button on canvas twice → code editor does NOT move on 2nd click (already on line)
• Rapid keyboard navigation (10 lines) → canvas syncs once after 150ms idle
• Switch to Code-Only mode → no canvas sync fires
• Switch back to Split mode → sync resumes on next caret move
• Undo/Redo after design operation → selection preserved, no feedback loop

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Known limitations / risk areas

• UID numbering uses raw XAML (not sanitized). If SanitizeForPreview ever removes entire elements (currently only strips attributes), UIDs would desync.
• Single-line compact XAML (<Grid><Button/></Grid>) always syncs to the last (deepest) element on the line — acceptable behavior.
• SelectionChanged subscription may fire during text drag-select; guarded by Selection.IsEmpty check.

[abbaye]

✅ Delivered — v0.6.0 (commits 73bca10 + 6e553bc)

Bidirectional canvas↔code selection sync shipped at ~95% fidelity:

• Canvas → Code: element click → locate UID in XAML source → position caret + highlight element tag
• Code → Canvas: caret position → parse element at offset → SelectElement() + adorner highlight
• 150 ms debounce on both directions (avoids feedback loop)
• XamlSourceLocationService handles UID injection and element mapping
• Error card overlay for XAML parse failures (does not block the designer)
• Zoom/scroll fixes also landed in same branch: always-visible scrollbars, ClampOffsets min>max guard, mouse wheel scroll restored

Branch DebugNewFeatures merged to master on 2026-03-19.