Feed: the wheel coast no longer stalls or reverses as pictures land. A row that painted before its picture's size was known (a cached row on a feed just opened, a link card, a clip learning its shape) grew after it was measured, and the stale height is what the spacer took when the row left the window: the page shrank by the difference, the browser's scroll anchoring pulled the reader back by as much, and a coasting scroll lost ground (or ran backwards) right as the media appeared — on a feed opened fresh, 18 such frames in four seconds. Rows are now watched by a ResizeObserver (after layout, nothing forced) so the heights stay live; originals decode async so a big one never holds a paint. Tried and dropped: shrinking no-CORS originals through createImageBitmap — it re-decodes on the main thread. tools/feed-scroll-jank.js grows a hands-off coast scenario (--coast: quick-notch bursts, then let go; dips/jumps in the coast's own travel next to what landed: media loads, layout shifts, row batches, page height, scrollTo/scrollBy callers), --trace (a Chrome trace: the browser's work inside each long frame — decode, layout, paint, GC — and the compositor's dropped-frame count), --url for a never-seen topic feed, --only settled|coast, --start px. Measured: the fresh-feed coast went from 18 dips to 8 small ones; a fresh topic feed coasts with one 33 ms frame in five seconds. Still open: a 9528×6328 original from a host without CORS (primal's) once froze a scroll step for 400 ms on its texture upload — only a resizing proxy fixes that one. feed-stability's "a small feed query starts before opening the feed" and "an unavailable image remains a usable link" fail on 491904e too.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

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