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Bug 1669865 - Fix invalidations from external scroll offset inaccuracy. r=kats

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Ensure that the external scroll offset is rounded from app units
into whole device pixels. This means that the primitive content id
matches in cases with non-round device pixel ratios, avoiding
unnecessary invalidations in these cases.

Differential Revision: https://phabricator.services.mozilla.com/D93175
This commit is contained in:
Glenn Watson 2020-10-12 22:11:00 +00:00
parent 6ea02af2f9
commit 1a7a9739e4
1 changed files with 9 additions and 2 deletions
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11
gfx/layers/wr/ClipManager.cpp
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@ -315,8 +315,15 @@ Maybe<wr::WrSpaceAndClip> ClipManager::DefineScrollLayers(
if (parent) {
parent->space = SpatialIdAfterOverride(parent->space);
}
LayoutDevicePoint scrollOffset =
metrics.GetLayoutScrollOffset() * metrics.GetDevPixelsPerCSSPixel();
// The external scroll offset is accumulated into the local space positions of
// display items inside WR, so that the elements hash (intern) to the same
// content ID for quick comparisons. To avoid invalidations when the
// auPerDevPixel is not a round value, round here directly from app units.
// This guarantees we won't introduce any inaccuracy in the external scroll
// offset passed to WR.
LayoutDevicePoint scrollOffset = LayoutDevicePoint::FromAppUnitsRounded(
scrollableFrame->GetScrollPosition(), auPerDevPixel);
return Some(mBuilder->DefineScrollLayer(
viewId, parent, wr::ToLayoutRect(contentRect),
wr::ToLayoutRect(clipBounds), wr::ToLayoutPoint(scrollOffset)));