External/buzz
1
0
Fork 0
mirror of https://github.com/block/buzz.git synced 2026-08-18 06:50:31 +02:00
Code Issues Packages Projects Releases Wiki Activity

Merge W1r3: hold reading row across WebKit realization via aboveShift fallback (dawn/webkit-compensation @ fe786ea4)

Browse Source 
Co-authored-by: Tyler Longwell <tlongwell@block.xyz>
Signed-off-by: Tyler Longwell <tlongwell@block.xyz>

* origin/dawn/webkit-compensation:
  fix(desktop): hold reading row across WebKit realization (engine-order-independent)
This commit is contained in:
npub1qyvc0c5kl4gqv2fd97fsk46tu378sqgy35vc83rvgfwne90sel7s0ed67d
2026-07-08 19:12:23 -04:00
co-authored by Tyler Longwell
parent 3d1e91ea03 fe786ea40e
commit b170680d8f
1 changed files with 40 additions and 18 deletions
Download Patch File Download Diff File Expand all files Collapse all files
desktop/src/features/messages/ui/useAnchoredScroll.ts
+40 -18
View File
@@ -649,27 +649,38 @@ export function useAnchoredScroll({
container.scrollTo({ top: container.scrollHeight, behavior: "auto" });
return;
}
// Mid-history: a batch of rows realized/reflowed this frame. The
// reading anchor — the first row a safe margin below the fold,
// snapshotted by the per-rAF baseline sampler — has shifted by the net
// height change of everything above it. Re-pin it to its saved offset: a
// single measured correction (the layout engine already summed the
// above-anchor deltas into the row's top, and rows resizing below the
// anchor don't move it, so they're excluded for free). This is the single
// scroll writer for realization; `overflow-anchor: none` (and WKWebView's
// absence of it) mean nothing else competes. We use the RO batch only as
// the TRIGGER — we detect that a row's laid-out height actually changed
// (seeded from the reserve so a first-realization delivery counts, not
// swallowed as a first sighting), then correct from the anchor's own
// measured shift, never from summing per-row deltas.
// Mid-history: a batch of rows realized/reflowed this frame. The reading
// anchor — the first row a safe margin below the fold, snapshotted by the
// per-rAF baseline sampler — has shifted by the net height change of
// everything above it, and must be re-pinned to its saved offset so it
// stays visually fixed. This is the single scroll writer for realization;
// `overflow-anchor: none` (and WKWebView's absence of it) mean nothing
// else competes. We use the RO batch as the TRIGGER — a row's laid-out
// height actually changed (seeded from the reserve so a first-realization
// delivery counts, not swallowed as a first sighting) — and correct via
// one of two engine-order-independent signals computed below.
let changed = false;
let aboveShift = 0;
const anchorForShift = readingAnchorRef.current;
const anchorRowForShift = anchorForShift
? container.querySelector<HTMLElement>(
`[data-message-id="${CSS.escape(anchorForShift.id)}"]`,
)
: null;
const anchorTopForShift = anchorRowForShift
? anchorRowForShift.getBoundingClientRect().top
: Number.POSITIVE_INFINITY;
for (const entry of entries) {
const row = entry.target as HTMLElement;
const height = row.getBoundingClientRect().height;
const rect = row.getBoundingClientRect();
const height = rect.height;
const last = lastHeights.get(row);
lastHeights.set(row, height);
if (last === undefined) continue; // never seeded (defensive).
if (Math.abs(height - last) > 0.5) changed = true;
// A row that grew/shrank ABOVE the anchor shifts the anchor by its
// height delta. Rows at/below the anchor don't move it.
if (rect.top < anchorTopForShift) aboveShift += height - last;
}
if (!changed) return;
const baseline = readingAnchorRef.current;
@@ -703,11 +714,22 @@ export function useAnchoredScroll({
scrollTop: currentScrollTop,
});
if (target !== null) {
// The anchor's own document position moved (Chromium: the rAF baseline
// is captured pre-reflow, so this fires and measures the exact shift).
container.scrollTo({ top: target, behavior: "auto" });
// No re-baseline here: the rAF loop is the single writer of
// `readingAnchorRef` and re-snapshots from live geometry next frame
// (~16ms), which already reflects this correction. Writing it here too
// would make the "single writer" invariant a lie for no gain.
} else if (Math.abs(aboveShift) > 0.5) {
// The position diff sees no move — the WebKit case, where the rAF
// baseline lands post-realization so `baseline` and `current` straddle
// the SAME side of the reflow (dTop=dScroll=0) and the diff is blind to
// it. The RO entries still carry the true magnitude: the net height
// change of the rows ABOVE the anchor is how far the anchor's document
// position shifted, so absorb it into scrollTop to hold the row. This is
// baseline-timing-independent — it reads the reflow directly rather than
// diffing a snapshot WebKit captured too late.
container.scrollTo({
top: currentScrollTop + aboveShift,
behavior: "auto",
});
}
});
// Observe every timeline row (not the content wrapper): a