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Bug 1549625 - Avoid clobbering the visual scroll offset if the layout viewport shrinks in size r=kats

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Differential Revision: https://phabricator.services.mozilla.com/D30434

--HG--
extra : moz-landing-system : lando
This commit is contained in:
Botond Ballo 2019-05-10 21:13:17 +00:00
parent 34c4308791
commit 965a150f32
1 changed files with 16 additions and 1 deletions
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17
layout/generic/nsGfxScrollFrame.cpp
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@ -2684,6 +2684,20 @@ void ScrollFrameHelper::ScrollToImpl(nsPoint aPt, const nsRect& aRange,
FrameLayerBuilder::GetPaintedLayerScaleForFrame(mScrolledFrame);
nsPoint curPos = GetScrollPosition();
// Below, we clamp |aPt| to the layout scroll range, compare the clamped
// value with the current scroll position, and early exit if they are the
// same. The early exit bypasses the update of |mLastScrollOrigin|, which
// is important to avoid sending a main-thread layout scroll update to APZ,
// which can clobber the visual scroll offset as well.
// If the layout viewport has shrunk in size since the last scroll, the
// clamped target position can be different from the current position, so
// we don't take the early exit, but conceptually we still want to avoid
// updating |mLastScrollOrigin| and clobbering the visual scroll offset.
bool suppressScrollOriginChange = false;
if (aPt == curPos) {
suppressScrollOriginChange = true;
}
nsPoint alignWithPos = mScrollPosForLayerPixelAlignment == nsPoint(-1, -1)
? curPos
: mScrollPosForLayerPixelAlignment;
@ -2759,7 +2773,8 @@ void ScrollFrameHelper::ScrollToImpl(nsPoint aPt, const nsRect& aRange,
// legitimate scroll offset updates because the origin has been masked by
// a later change within the same refresh driver tick.
allowScrollOriginChange =
mAllowScrollOriginDowngrade || !isScrollOriginDowngrade;
(mAllowScrollOriginDowngrade || !isScrollOriginDowngrade) &&
!suppressScrollOriginChange;
if (allowScrollOriginChange) {
mLastScrollOrigin = aOrigin;