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[pull] master from mozilla:master #507

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Merged
pull merged 2 commits into from
Aug 18, 2026
+24 −27
Merged

[pull] master from mozilla:master #507

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e31de7e
Improve the `watchScroll` helper function a little bit
Snuffleupagus Aug 18, 2026
0adc2e7
Merge pull request #21796 from Snuffleupagus/watchScroll-improve
timvandermeij Aug 18, 2026
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51 changes: 24 additions & 27 deletions web/ui_utils.js
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Original file line number Diff line number Diff line change
Expand Up @@ -117,43 +117,40 @@ function scrollIntoView(element, spot) {
* PDF.js friendly one: with scroll debounce and scroll direction.
*/
function watchScroll(viewAreaElement, callback, abortSignal = undefined) {
const debounceScroll = function (evt) {
if (rAF) {
return;
function onRAF() {
rAF = null;

const currentX = viewAreaElement.scrollLeft;
const lastX = state.lastX;
if (currentX !== lastX) {
state.right = currentX > lastX;
}
// schedule an invocation of scroll for next animation frame.
rAF = window.requestAnimationFrame(function viewAreaElementScrolled() {
rAF = null;

const currentX = viewAreaElement.scrollLeft;
const lastX = state.lastX;
if (currentX !== lastX) {
state.right = currentX > lastX;
}
state.lastX = currentX;
const currentY = viewAreaElement.scrollTop;
const lastY = state.lastY;
if (currentY !== lastY) {
state.down = currentY > lastY;
}
state.lastY = currentY;
callback(state);
});
};
state.lastX = currentX;
const currentY = viewAreaElement.scrollTop;
const lastY = state.lastY;
if (currentY !== lastY) {
state.down = currentY > lastY;
}
state.lastY = currentY;
callback(state);
}

const state = {
right: true,
down: true,
lastX: viewAreaElement.scrollLeft,
lastY: viewAreaElement.scrollTop,
_eventHandler: debounceScroll,
};

let rAF = null;
viewAreaElement.addEventListener("scroll", debounceScroll, {
useCapture: true,
signal: abortSignal,
});
viewAreaElement.addEventListener(
"scroll",
() => {
// Schedule an invocation of scroll for next animation frame, when needed.
rAF ??= window.requestAnimationFrame(onRAF);
},
{ useCapture: true, signal: abortSignal }
);
abortSignal?.addEventListener(
"abort",
() => window.cancelAnimationFrame(rAF),
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