Affected versions

• Firefox 67.0b5 (20190325125126)
• Firefox 68.0a1 (20190327044526)
• Firefox 66.0.2 (20190326175229)

Affected platforms

• macOS 10.14
• Ubuntu 18.04 x64
• Windows 7 x64
• Windows 10 x64

Steps to reproduce

1. Open Firefox and go https://www.reddit.com/.
2. Drag the scrollbar or use touchpad to fast scroll up and down on the page.
3. Observe the behavior.

Expected result

• The scrolling is smooth without any visual glitches.

Actual result

• "Back to top" button flashes.
• The ADS section flashes as well.

Regression range

• Last good revision: 3119a9a0b5dee60ac77b7596ae5dbe0658f598ad (2016-12-27)
• First bad revision: d7b6af32811bddcec10a47d24bd455a1ec1836fc (2016-12-28)
• Pushlog:
  https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=3119a9a0b5dee60ac77b7596ae5dbe0658f598ad&tochange=d7b6af32811bddcec10a47d24bd455a1ec1836fc

Additional Notes

• On Chrome the advertisement doesn't flash on scroll, neither the "Back to top" button.
• The issue is more visible on macOS where the touchpad is used to fast scroll up and down.
• Attached a screen recording with the issue: link.

Updating summary and adding regression range.

Could this be APZ related, Kats?

Yeah this looks like it's another instance of bug 1424714.

Is this old or new reddit?

Reddit is currently undergoing a redesign, and the new design uses endless scrolling, so of course it's buggy.

I use my own font settings so I can read things, and sites which constantly load and unload elements end up flashing as they load elements based on their expected sizes, and then resize elements based on my sizes.

Hello, this is on new reddit. The issue doesn't apply on old one because on scrolling the ads don't follow on scrolling on the page. And "Back to top" button is not present there. I tried setting the page to display more content but still the presented behavior don't apply. Thank you!

Bulk change of P3 carryover bugs to wontfix for 68.

Happy to take a patch for 70 but since this is triaged and set to P3 priority I'm setting it as fix-optional.
That will remove the bug from weekly regression triage.

Alexandru, would you be able to re-test with a recent Nightly, to see if this was fixed by bug 1424714?

The flashes are still happening using the latest Nightly (20200422214848) on Windows 10x64. I made a screen recording: link.

Though on my Linux box, I don't see any flickers on reddit.com with an older version of nightly (2021-08-01). It looks like reddit has changed the header position style to "fixed" rather then "sticky"?

Anyways I am going close this now.

Oh right. Bug 1714569 noticed me that a sitcky element is not the header, it's actually two boxes at the right side of the page. They are flickering on the older version, they no longer flickers on the latest nightly (buildid: 20211014212856).

Confirming that the flicker no longer occurs with Firefox 95.0b2 (20211102190739) on Windows 10x64, macOS 10.15, and Ubuntu 21.04. The only thing is that only one box is present on the right side and not two.

On MacOS 10.14.6, using https://archive.mozilla.org/pub/firefox/nightly/2021/11/2021-11-02-21-35-08-mozilla-central/firefox-96.0a1.en-US.mac.dmg with color, font, etc. changes for readability, hardware acceleration disabled for readability, etc.

https://old.reddit.com/ and https://new.reddit.com/ both flash if I scroll. Worse than before. New also puts a non-scrolling column next to a scrolling one, resulting in awful migraines.

i've switched colors and fonts for readability. Note that the upper and lower edges of that section flash.

(In reply to MarjaE from comment #14)

Created attachment 9249362 [details]
Screen Recording 2021-11-04 at 11.09.06 PM.mov

i've switched colors and fonts for readability. Note that the upper and lower edges of that section flash.

I guess I'm not sensitive to this, I haven't been able to see a flash. Can you describe in more detail what I should look for? The top/bottom lines of the rounded rectangle in the centre of the video?

Yes. I get intense flashing when I try to watch the screenshot.

MarjaE, can you please open a new bug for the flash you are seeing? It seems to me that it's kinda non-smooth scrolling rather than flashing?

Or it's the same symptom of bug 1735963.