Match transparent canvas backgrounds to the active theme

[jonathanKingston]

Transparent inline canvases were composited against different surfaces: the visible webview exposed Copse's active theme, while the hidden agent mirror defaulted to white. A dark-theme canvas could therefore disagree with its preview even though both loaded the same artifact bytes.

This change samples Copse's resolved body background (including UI tint) and uses it as the hidden BrowserWindow's compositor backdrop. The visible webview host explicitly uses the same base theme surface. Artifact CSS remains authoritative, so an artifact that paints its own background still overrides the default; a fully transparent sampled color falls back to the resolved boot theme.

The focused Electron regression emits both a transparent artifact and one with an explicit background. It checks the preview pixel against the active tinted theme, checks the open canvas host against the app surface, verifies the explicit artifact color wins, and saves the transparent dark-theme state for review.

Validation:

• pnpm test -- canvas-agent-mirror — 11 passed
• pnpm run build — passed
• Typecheck, type-aware lint, formatting, demo-site consistency, and dead-code analysis — passed after the final edits
• pnpm run check:oracle — 262 specs live, 15 invariants passed
• pnpm run check:e2e-syntax — 289 files parsed cleanly

The full pnpm run check could not complete its unit phase in this command sandbox: unrelated watcher tests receive EMFILE, fixed /tmp paths receive EPERM, and a later durable rerun stalled behind the inaccessible orphaned test process. The focused local Electron spec also stopped before Copse launched because Chromium could not create its extension-unpack temp directory. CI should run tests/e2e/canvas-background-parity.e2e.ts and publish the visual evidence.

Co-Authored-By: Copse noreply@copse.dev
Copse-Models: acp:codex-acp#gpt-5.6-sol, acp:claude-acp#claude-fable-5-1[1m]

[github-actions]

🖥️ PR preview

• Marketing site
• Browser demo
  Interactive, with a mocked backend — jump to a scenario:
• Builds a cupcake site
• Agent-proposed edits open the Changes panel
• Markdown list indentation
• Unattended container run
• Responsive composer footer
• Subagent display visual reference
• Subagent stays ungrouped beside reading tools
• Semantic search subagent markdown
• Light-theme approval with a bright accent
• Light-theme syntax, fills, and selection
• Grouped outside-sandbox command approval
• Remote desktop discovered-port list with one selected
• Settings scroll + sticky footer geometry
• Per-model reasoning / temperature / top-p
• Thread switch hydration notice
• Thread switch hydration failure
• Apple Development test profile
• Chat layout styling
  _{Static demo of the renderer against a mock API — the browser pane, terminal, and all main-process IPC are stubbed, so it never proves main-process behaviour. Rebuilt whenever a push changes the demo bundle; these links go live once the queued deploy lands, shortly after this comment. Removed when the PR closes.}

[copse-release-bot]

Reference screenshots ready for review

Review GitHub’s image diffs in screenshot PR #2710.
Merge it (or enable auto-merge) to apply the accepted PNGs to copse/from-id-be0d523a-d366-449e-ad4b-ae568a0f13-1e2561.

Rendered for eb434cc8bbd1 by CI run 35477648789; the immutable artifact is reference-screenshot-candidates-35477648789.

If this source branch moves, a later successful render closes the stale review PR and replaces this link.