V3: transparent borders for newly contained media

[git-chad]

New image and video layers in Contain mode now reveal lower content at their empty borders instead of painting black bars. A Transparent Borders toggle, shown for Contain mode, restores black borders when disabled. Cover mode is unchanged.

Saved projects without the setting are normalized to transparentBounds: false before applying new defaults. Existing exported runtime configs also retain black borders when the setting is absent. Both editor and runtime MediaPass use the same opt-in alpha behavior; shader export preserves the setting.

Stack: this PR targets git-chad/v3-composition-alpha (#151), which targets git-chad/shader-lab-v3-plan (parent #150). Nothing in the stack targets main except the draft parent PR. The roadmap and CI branch filters now support this workflow.

Validation:

• bun run test:composition: all legacy PNG baselines and 136 editor/runtime blend samples pass unchanged; added GPU coverage for image/video borders, opacity, scale/offset, Cover behavior, migration, save/reopen, shader export, and PNG export.
• bun run check, runtime typecheck, and MCP typecheck pass (three pre-existing app lint warnings).
• Includes a runtime patch changeset and transparent/solid PNG comparison artifacts in the composition CI job.

This is a bounded first fix in roadmap step 1.2. It does not implement transparent scene/export backgrounds, isolated groups, or new mask semantics.

Manual preview check:

1. Add a colorful background layer and an image above it; choose Contain and reduce Scale if necessary to expose its borders.
2. Confirm the background shows through the borders while the image stays opaque. Turn Transparent Borders off and confirm the borders become black; turn it back on.
3. Save and reopen, then export a PNG. Confirm the selected border behavior persists. An old saved project should retain its original borders.

[vercel]

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Project	Deployment	Actions	Updated
shader-lab	Ready	Preview	Sep 17, 2026 2:14am UTC

[greptile-apps]

Greptile Summary

This PR adds opt-in transparent borders for contained image and video layers while preserving opaque borders for legacy projects and runtime configurations.

• Adds the editor setting and matching editor/runtime rendering behavior.
• Migrates missing saved-project values to the compatibility default.
• Preserves the setting through save/reopen and shader export.
• Adds GPU composition coverage and enables CI for stacked V3 branches.
• Updates runtime documentation, roadmap guidance, and release metadata.

Confidence Score: 5/5

The PR appears safe to merge with no actionable correctness, security, or repository-rule violations identified.

Transparent bounds compose correctly in both renderers, absent values preserve compatibility, new layers receive the intended default, and persistence and export paths retain explicit values.

_{Reviews (1): Last reviewed commit: "fix: reveal lower content around newly c..." | Re-trigger Greptile}