Buttons used to ignore `isMask` on every render path — the mask
shape drew as normal content instead of clipping the button's
other layers. Every entry point now routes button children through
`drawMaskedGroup`:
- `drawLayers` nested-button branch
- new `drawButtonFrame` (used by the mask-preview overlay in
button-edit mode so the preview reflects the button's own
stage instead of the main scene)
- `renderButtonThumbnail` so sidebar previews match the stage
- exported-runtime button branch mirrors the split; bumped
`RUNTIME_VERSION` to '11' so cached players reload
Also:
- Timeline clip-track dropzone hides until the library has at
least one clip, and the hint stays centred in the visible
ruler area instead of drifting with horizontal scroll.
- Regression tests pin positive/negative button masks in
`drawButtonFrame` and verify `drawSceneFrame` applies
button-internal masks when the button sits on the main stage.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
thandClaude Opus 4.7
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