fix: canvas corruption in dispose-to-background for non-alpha frames#178
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The clear code in `composite_frame` branched on `frame_has_alpha` to decide byte stride, but the canvas is always RGBA (4 bytes/pixel). The `!frame_has_alpha` paths used 3-byte stride — `chunks_exact_mut(3)` for full-size and `* 3` indexing for sub-frames — misaligning pixel writes and corrupting the canvas. This triggers when an animation has `dispose=Background` and the next frame lacks alpha (e.g. lossy VP8 without ALPH chunk). The previous frame's rectangle gets cleared with wrong byte offsets. Fix: always clear with 4-byte pixels. `frame_has_alpha` describes the incoming frame data format, not the canvas format. Adds unit tests for both full-size and sub-frame disposal with RGB frames. The sub-frame test fails against the old code with: pixel (4,3) not cleared: [0, 0, 170, 170]
fintelia
approved these changes
Mar 25, 2026
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Summary
composite_frameclear code branched onframe_has_alphato pick byte stride, but the canvas is always RGBA (4 bytes/pixel)!frame_has_alphapaths used 3-byte stride (chunks_exact_mut(3)for full-size,* 3indexing for sub-frames), misaligning pixel writesdispose=Backgroundand the next frame lacks alpha (e.g. lossy VP8 without ALPH chunk)Test plan
dispose_clear_fullsize_rgb_frameunit testdispose_clear_subframe_rgb_frameunit test (confirms failure on old code:pixel (4,3) not cleared: [0, 0, 170, 170])