Fenix: [Bug] Flickery white line at bottom of page - Firefox Nightly Android

[st3fan]

Taken from https://bugzilla.mozilla.org/show_bug.cgi?id=1634969

(Images in Bugzilla bug)

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:76.0) Gecko/20100101 Firefox/76.0

Steps to reproduce:

Open a website with a dark background colour in Firefox Nightly for Android - using Oneplus 7 Pro. OxygenOS 10.0.5 based on Android 10. Nightly version 200502 06:01

Actual results:

As I scroll down I see a flickering white line at the bottom of the screen, maybe 1 or 2 pixels high.

Expected results:

No line

[ekager]

This sounds like a toolbar margin issue maybe?

[ekager]

Seeing a tiny 1 pixel line at the top of the screen with bottom toolbar on today's nightly.

[liuche]

Need to be in dark theme to notice.

(have top toolbar)
Force-quit app
load github, scroll around a little

sometimes it comes and goes.

[liuche]

E2 timebox

[Tajnymag]

I've recorded a screencast showing the bug in motion.

[kglazko]

I've been able to reproduce this issue (I think- or else something similar looking) on Reddit. The difference is that when you scroll the webpage, you can actually see contents of the web page such as logos or text show up within the white bar. I'm attaching a screenshot and will continue to investigate tomorrow, but now I'm wondering if this is a web content issue of some kind..

[liuche]

We've added some custom behaviors on top and bottom dynamic toolbars, so this might be a side effect, and something that we're telling GV. This might be something that happens because we're telling GV the wrong value in setVerticalClipping.

Affects Reddit, etc

@Mugurell do you have any ideas about something that might be causing this "off by 1 pixel" problem?

[Mugurell]

This seems pretty bothersome though at first glance nothing stands out in code..

Trying to find a regression range got me so far that I had to use builds for FirefoxCI because locally there were issues with building off of old revisions.

I'm seeing the issue reproducing even on the oldest build available on CI (Nov 2019), even before the dynamic toolbars - https://drive.google.com/file/d/1X0zA4vVcKlFZtJ62qdf06S_-KmM2lwLn

Will investigate a bit more.

[Mugurell]

I think the white space is actually rendered by GV.
Filed https://bugzilla.mozilla.org/show_bug.cgi?id=1648079 for this.

[kglazko]

Thank you! I suspected that too :) I will make sure it is on their radar.

[michas0602]

Same problem here. Found on my own hp, so I could play a bit in CSS and so on if anyone has an idea how to work around or find out why this happens.

Firefox Beta
84.0.0-beta.4 (Build #2015780083)
AC 67.0.9
GV:84.0-20201206192040
AS: 67.0.0

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