Findings No. 7
A canvas wider than the GPU composites shows nothing, though it draws
Drawing succeeds and getImageData reads back fine, but the compositor drops textures past its limit (commonly 8,192 px). Draw only the window in view and follow the scroll.
A canvas wider than the GPU's texture limit draws successfully and composites to nothing. Our clip waveforms spanned the whole clip: a two-and-a-half-minute stem on a Retina screen is 11,500 backing pixels at default zoom and 39,000 zoomed in. Drawing into that canvas works, and getImageData reads the bars back, so headless probes at scale factor 1 saw nothing wrong, while on a real Mac the clips rendered blank: the compositor drops textures past its limit, commonly 8,192 pixels, with 32,767 as the hard cap.
Capping the backing store shows the bars but blurs them on a Retina screen. The real fix is to draw only the window in view, at full resolution, and follow the scroll.
Receipt: Stem Shovel (clipWave, StudioTimeline, 2026-10-20).