1375384 - Glitches on top and bottom of webpages when scrolling

Status: RESOLVED DUPLICATE of bug 1373335

(Core :: Panning and Zooming, defect)

Description

[Bogdan Maris, Desktop Test Engineering]

[Affected versions]:
- Firefox 56.0a1

[Affected platforms]:
- macOS 10.12

[Steps to reproduce]:
1. Start Nightly
2. Visit a page with content so the scrollbar appears 
eg: https://www.mozilla.org/en-US/firefox/sync/
3. Scroll through the page

[Expected result]:
- No artifacts can be seen while scrolling.

[Actual result]:
- One pixel line can be seen while scrolling on top of webpages. 

[Regression range]:
- This is a recent regression. Here is the pushlog from mozregression (not sure which bug is the culprit though):

https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=958d2a5d10091401fd5e900e8e063d21940c137e&tochange=7f894f791cdf170d788507d0eff30024ce699523

[Additional notes]:
- Setting 'layers.async-pan-zoom.enabled' to 'false' will make this issue disappear.
- This can also be seen inside Developer Tools (inspector, storage etc...) when scrolling.
- Here is a screencast showing the issue (it was to big to upload it here directly): https://www.dropbox.com/s/79war3f8jk8u4rd/Scroll%20glitch%20apz.mov?dl=0
- Windows and Linux are not affected by this, only Mac.