fix(desktop): send host theme to built canvas artifacts

[charlesvien]

Problem

Published canvases render in light mode inside a dark app. Dark mode stopped working when canvases moved to cloud-built artifacts (#76649): the artifact path lost the theme bridge that edit mode still has.

• The BuiltCanvas host never sends the theme over the artifact bridge.
• The runtime baked into built artifacts never applies one. fix(canvas): apply host theme in built artifact runtime #78694 fixes that half; the desktop backend coupling check requires the split, and the halves are safe to ship in either order.

Changes

• BuiltCanvas posts a set-theme frame over the bridge port after connect and on every host theme change, matching FreeformCanvas.
• The mount-time theme rides a #theme= fragment on the artifact URL, so the new runtime themes the first paint instead of waiting for the port. Fragments do not reach the server, so signed URLs stay valid.
• The host iframe gets color-scheme, so the UA does not paint the embedded canvas white before the artifact loads.

Note

Existing artifacts embed the old runtime and stay light until their canvas rebuilds with #78694 deployed.

No screenshot: I did not run the desktop app against a real built artifact.

How did you test this code?

• New BuiltCanvas test: the srcDoc carries the theme fragment, the iframe carries color-scheme, and the bridge port receives a dark frame on connect and a light frame on theme change. Catches the host dropping theme wiring again.
• Ran the freeform canvas Vitest suite and @posthog/ui typecheck locally.
• Not checked: the running app with a real built artifact.

Automatic notifications

• Publish to changelog?

Docs update

None.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Claude Code session. Skills invoked: canvas-templates, writing-tests, writing-pr-descriptions. I traced the regression to the cloud artifacts port, which swapped the theme-aware sandbox for the artifact path on published builds. Originally one PR with the builder half; split into #78694 when the desktop backend coupling check failed. The pre-paint fragment and iframe color-scheme address a Greptile review comment about the light flash before the bridge connects.

[trunk-io]

😎 Merged successfully - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 54d2833.}

[greptile-apps]

Prompt To Fix All With AI

### Issue 1
products/desktop/packages/ui/src/features/canvas/freeform/BuiltCanvas.tsx:170-174
**Cover the initial color scheme**

The initial theme is delivered only after the artifact runtime starts its message port, so a dark-themed artifact can first paint its background, native controls, or scrollbars using the light color scheme. Apply the resolved scheme at the iframe level as FreeformCanvas does to cover the pre-runtime paint.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "fix(canvas): mirror host dark mode into ..." | Re-trigger Greptile}

[trunk-io]

   

View Full Report ↗︎ ⋅ Docs

[stamphog]

Contained desktop-UI theming fix (iframe/message-bridge only, no schema/API/auth/CI/dependency surface); it adds test coverage and the diff directly addresses Greptile's resolved review comment about the pre-paint color-scheme flash via the new iframe colorScheme style.

• 👍 on the PR from greptile-apps[bot], hex-security-app[bot].

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✓	no deny categories matched
size	✓	69L, 1F substantive, 137L/2F incl. docs/generated/snapshots — within ceiling
tier	✓	T1-agent / T1c-medium (137L, 2F, single-area, fix)
stamphog 2.0.0b4		.stamphog/policy.yml @ ada83d8 · reviewed head 54d2833

[charlesvien]

/trunk merge

[deployment-status-posthog]

Deploy status

Environment	Status	Deployed At	Workflow
dev	✅ Deployed	2026-08-06 08:09 UTC	Run
prod-us	✅ Deployed	2026-08-06 08:26 UTC	Run
prod-eu	✅ Deployed	2026-08-06 08:28 UTC	Run