+++ This bug was initially created as a clone of Bug #1527899 +++

Build ID 20190215045130
User Agent Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:67.0) Gecko/20100101 Firefox/67.0

STR:
0. WebRender turn on and restart

1. Launch Firefox.
2. Navigate to testcase of Bug 1519553 attachment 9036063 [details]
3. Scroll up by 20-30px with drag scrollbar thumb
   or Scroll up or down with mouse wheel one notch
4. Observe

(optionally if not reproduced)
5. Repeat step3

ER: Solid scroll position
AR: The contents jitter.

Taking since I'm looking at this for the moment. Andrew was also looking at more general snapping, so this may also be interesting to him.

So I did a bit of debugging. What's going on is:

• Scroll anchoring adjusts to the offset of the frame.
• APC requests a fractionally different offset:

(rr) p geckoScrollPosition
$7 = {<mozilla::gfx::BasePoint<float, mozilla::gfx::PointTyped<mozilla::CSSPixel, float>, mozilla::gfx::CoordTyped<mozilla::CSSPixel, float> >> = {{{x = 0, y = 290.683319}, components = {0, 
        290.683319}}}, <mozilla::CSSPixel> = {<No data fields>}, <No data fields>}
(rr) p targetScrollPosition
$8 = {<mozilla::gfx::BasePoint<float, mozilla::gfx::PointTyped<mozilla::CSSPixel, float>, mozilla::gfx::CoordTyped<mozilla::CSSPixel, float> >> = {{{x = 0, y = 290.68335}, components = {0, 
        290.68335}}}, <mozilla::CSSPixel> = {<No data fields>}, <No data fields>}

• The APC scrolling ends up in ScrollToImpl, where we change the offset, invalidate the anchor, and start again.

This position is only invalidated from FrameLayerBuilder, that's not a very good
signal.

This would get out of sync when using WebRender, and APZ messages will send
sub-pixel scroll positions that will get wrongly adjusted, causing small scroll
offsets which scroll anchoring would incorrectly try to correct, which is the
ultimate cause of the jittering.

Mentioned this on IRC but: I'm only mildly confident in https://phabricator.services.mozilla.com/D37875 being the 100% correct fix, but I'm pretty sure that what we're doing right now is completely wrong. I'm unsure what the WebRender equivalent to the two calls of https://searchfox.org/mozilla-central/rev/15be167a5b436b57fef944b84eef061d24c1af8c/layout/painting/FrameLayerBuilder.cpp#2550 are, but an alternative that probably also works is calling that from the WebRender equivalents.

(It is unclear there is a WebRender equivalent at all to the point where we create a layer for a scrollable frame. :))

Try looks good btw: https://treeherder.mozilla.org/#/jobs?repo=try&revision=ba18d6285d64968b1379ca41c5145784d797ece7

https://hg.mozilla.org/mozilla-central/rev/a37238f046d5

Bugbug thinks this bug is a regression, but please revert this change in case of error.

Comment on attachment 9077691 [details]
Bug 1528180 - Don't align scroll offsets to layer pixels when using WebRender. r=kats,jrmuizel

Beta/Release Uplift Approval Request

• User impact if declined: Bug 1554861 + jittering in some pages.
• Is this code covered by automated tests?: No
• Has the fix been verified in Nightly?: Yes
• Needs manual test from QE?: Yes
• If yes, steps to reproduce: comment 0
• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): Very low effort patch that fixes a WR correctness bug. The risk of this change having fallout is pretty low I'd say.
• String changes made/needed: none

Comment on attachment 9077691 [details]
Bug 1528180 - Don't align scroll offsets to layer pixels when using WebRender. r=kats,jrmuizel

Fixes a scroll anchoring bug with WebRender. Approved for 69.0b6.

https://hg.mozilla.org/releases/mozilla-beta/rev/a60290b9de2c

Verified as fixed on Firefox 70.0a1 (2019-07-17) on Windows 10 x 64, Windows 7 x32, Mac OS X 10.14 and on Ubuntu 18.04 x64.

Verified as fixed on Firefox 69.0b6 on Windows 10 x 64, Mac OS X 10.14 and on Ubuntu 18.04 x64.