User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0

Steps to reproduce:

This issues is repeatable 100% of the time with ALL of the following:
Using a 4k display
Using Gnome3 (X11)
Fractional Scaling enabled (And not set to greater than 100%)
Firefox in full screen

The issue does not appear if ANY of the following:
Firefox is less than full screen
Using Gnome3 (Wayland)

This behaviour is present in both the Flatpak and .deb (apt) versions of Firefox.

Actual results:

Scrolling down with the mouse has an input delay of ~0.5 seconds.

The delayed scrolling does not appear in any other applications, including chromium on the same web pages.

Expected results:

Usually in Firefox there is not a visible delay when scrolling.

Corrections the line "Fractional Scaling enabled (And not set to greater than 100%)" should be "Fractional Scaling enabled (And set to greater than 100%)"

Bugbug thinks this bug should belong to this component, but please revert this change in case of error.

Hey Brock, thanks for reporting.

Could you:

• Attach your about:support information
• If possible, capture a profile using https://profiler.firefox.com/ and publish/paste the link

Thanks!

Here is the captured profile: https://share.firefox.dev/38Jq7UW
The about:support raw data and text are attached.
Hope this helps! Let me know what else you might need.

@aosmond: Could you see take a look at what's going wrong here?

Given your setup, I imagine we are hoping to push WebRender out to users with a similar configuration in the next few releases. Could you try turning on WebRender and letting us know how performance is? If it isn't good, collect a profile. Thanks! You can force WebRender on via about:config and setting gfx.webrender.all to true.

Also if the perf is bad, would you mind collecting the profile with nightly? That will have the most useful information for us.

I had to change to a different distro, because Pop!_OS was having performance issues and crashing issues other than just this Firefox bug. Though I did a live boot of Pop!_OS and I was able to replicate the issue despite the change.

Anyway, setting webrender.all to true helped a ton! Firefox went from totally unusable to very good! Thanks! Here is the profile: https://share.firefox.dev/2OmheqX

Performance was not perfect. When scrolling slowly there was no obvious delay. Though with quick scrolls the delay was quite obvious. Here is the profile for that on Nightly: https://share.firefox.dev/30aoPhF

(In reply to Brock from comment #10)
Has this been fixed by WebRender or should this bug stay open?

I no longer have access to the hardware that the issue occured on. So I honestly have no idea, sorry. I'm fine with this being closed though. Thank you for reaching out!