perf(canvas): detach hidden layers from DOM, throttle onscreen check, increase cache

[Pfannkuchensack]

Summary

This is Step 1 of the Canvas Optimization plan.

• setVisibility now uses layer.remove()/stage.add() to detach/reattach hidden canvas elements from the DOM, freeing browser compositing resources
• Wrap syncIsOnscreen and syncIntersectsBbox with rafThrottle to reduce redundant calculations during pan/zoom
• Increase canvasElementCacheSize 32→128 and imageDataCacheSize 32→64
• Add 3-Layer-Flattening design document for future architectural optimization

Related Issues / Discussions

Performance degrades significantly with many raster layers on the canvas. Each Konva.Layer creates a separate HTML <canvas> element — with 20+ layers, the browser compositor struggles under the load.

QA Instructions

1. Create 15+ raster layers on the canvas
2. DOM check: Open Browser DevTools → Elements → verify that only visible/on-screen layers have <canvas> elements in the DOM
3. Pan/Zoom: Pan and zoom the canvas — should feel smoother than before, especially with many layers
4. Layer switching: Select different layers — verify they appear correctly with proper z-order
5. Enable/Disable: Toggle layers on/off — verify they show/hide correctly
6. Type visibility: Hide/show entire layer types (e.g. all raster layers) — verify correct behavior
7. Isolated preview: Use filter/transform on a layer with isolated preview enabled — verify other layers hide/show correctly
8. Generation: Run a generation with multiple layers — verify compositing still works correctly

Merge Plan

No special merge considerations. No Redux slice changes, no DB schema changes.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[dunkeroni]

Had some trouble testing this at first. On Firefox specifically (and possibly in the electron app) this does improve performance for hidden raster layers. On Chrome, some caching witchcraft makes it fast all the time as long as the layer data does not change, so this change is not noticeable.