Move canvas background resolution from the LayoutBox level to the
LayoutTree itself, so the display list builder can paint it as a
full-viewport rect before any layout boxes. This fixes two issues:
1. Canvas background now covers the full viewport instead of just
the root element's border box
2. Background resolution works from computed styles (Phase 1) so it
respects display:none/contents on root and body per CSS
Backgrounds 3 §2.11.2
Phase 2 clears the root and (if propagated) body LayoutBox backgrounds
to prevent double-painting.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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