Add a checkered background to transparent artboards and the infinite canvas

[Keavon]

Artboards with a transparent background color, and the infinite canvas, now display a checkerboard pattern to indicate where transparency is present. Now, instead of displaying a white background and relying on the Export dialog's Transparency checkbox to disable that background just during the export, we support a proper transparent background for both artboards and the infinite canvas. That checkbox has been removed to simplify the export process so it matches expectations of what is shown. Creating a new document with an infinite canvas now includes a locked white solid color layer labeled "Background", adding the capability of altering that background color when it previously displayed always as white.

Closes #4021

[gemini-code-assist]

Code Review

This pull request removes the explicit "Transparent Background" option from the export dialog, implementing automatic transparency handling and a checkerboard pattern for transparent areas in the viewport. It also defaults artboard clipping to true and adds a white background layer for new documents without artboards. Review feedback identifies a bug in the JPG blending logic concerning premultiplied alpha and a potential compilation error in the checkerboard generation code.

[cubic-dev-ai]

2 issues found across 21 files

Confidence score: 3/5

• Moderate merge risk: there is a concrete behavior bug in editor/src/messages/dialog/new_document_dialog/new_document_dialog_message_handler.rs (severity 6/10, high confidence) where finite zero-dimension documents can be misrouted into infinite-canvas background initialization.
• node-graph/nodes/gstd/src/render_node.rs has a rendering-stability risk (severity 5/10) because the Vello checkerboard path divides by viewport_zoom without a positive-value guard, so zero zoom can produce invalid Affine scale values (infinite/NaN).
• These are specific, user-facing edge-case regressions rather than broad architectural problems, so this looks fixable but carries some real runtime risk until patched.
• Pay close attention to editor/src/messages/dialog/new_document_dialog/new_document_dialog_message_handler.rs and node-graph/nodes/gstd/src/render_node.rs - document initialization routing and zero-zoom rendering guards need validation.

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="editor/src/messages/dialog/new_document_dialog/new_document_dialog_message_handler.rs">

<violation number="1" location="editor/src/messages/dialog/new_document_dialog/new_document_dialog_message_handler.rs:34">
P2: Finite zero-dimension documents are incorrectly routed through the infinite-canvas background initialization branch.</violation>
</file>

<file name="node-graph/nodes/gstd/src/render_node.rs">

<violation number="1" location="node-graph/nodes/gstd/src/render_node.rs:211">
P2: Vello checkerboard path divides by viewport_zoom without ensuring it’s positive, unlike the SVG branch. A zero zoom would yield an infinite/NaN Affine scale and can corrupt rendering; add the same `viewport_zoom > 0.` guard here.</violation>
</file>

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