helper_relative_scroll_update_negative_offset.html

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<!DOCTYPE HTML>

<meta charset="utf-8">

<script src="apz_test_utils.js"></script>

<script src="/tests/SimpleTest/paint_listener.js"></script>

<style>

#scroller {

  overflow: scroll;

  width: 300px;

  height: 300px;

#content {

  height: 3000px;

</style>

<div id="scroller">

  <div id="content"></div>

</div>

<script>

"use strict";

// The scenario:

//   APZ performs a smooth scroll toward the top of a subframe.  While the

//   animation is in progress, the main thread is a frame or two behind APZ's

//   actual visual scroll position.  If a large instant scrollBy() that overshoots

//   the top boundary fires during this window, the main thread builds a

//   Relative ScrollPositionUpdate whose delta is large enough that, when APZ

//   applies it to its current (advanced) visual offset, the result is negative.

//

// Without the fix in FrameMetrics::ApplyRelativeScrollUpdateFrom the visual

// scroll offset in APZ would go negative, producing a blank gap at the top of

// the scroll container and a hit-test misalignment equal to the gap height.

async function test() {

  const scroller = document.getElementById("scroller");

  scroller.scrollTop = 500;

  await promiseApzFlushedRepaints();

  const scrollendPromise = promiseOneEvent(scroller, "scrollend");

  scroller.scrollTo({ top: 0, behavior: "smooth" });

  // Wait for APZ to advance and deliver one repaint, updating mApzScrollPos

  // to some position Q while APZ has already moved past Q toward 0.

  await promiseOneEvent(scroller, "scroll");

  await promiseFrame();

  // The main thread emits NewRelativeScroll(source=Q, dest=0, delta=-Q).

  // APZ applies: visual_offset(< Q) + (-Q) < 0 without the fix.

  scroller.scrollBy({ top: -1000, behavior: "instant" });

  await scrollendPromise;

  await promiseApzFlushedRepaints();

  // All APZ-sampled scroll offsets accumulated during the scroll must be >= 0.

  const sampledData = collectSampledScrollOffsets(scroller);

  ok(!!sampledData.length, "should have at least one sampled APZ offset");

  for (const data of sampledData) {

    const offsetY = SpecialPowers.wrap(data).scrollOffsetY;

    ok(offsetY >= 0,

       `APZ scroll offset must not be negative (got ${offsetY})`);

  is(scroller.scrollTop, 0, "final scroll position should be at the top");

waitUntilApzStable()

  .then(test)

  .then(subtestDone, subtestFailed);

</script>