[Platform affected]: Windows 10
[Environments]: FF beta

Preconditions:

• make sure that the webrender is enabled by checking in about:support/Graphics -Compositing.

Steps to reproduce:

1. Access a beta build (70.0b5 -version tested).
2. After opening FF browser click on the maximize button (FF should be in reduced size).
3. Access any website (for ex: reddit.com/youtube.com)
4. Reduce the browser size by dragging a window side border and inspect it.

Expected results: The browser should be resized properly, no side effect should occur.

Actual results: The browser has a flickering/trembling effects -can be seen in the video attached.

Notes:

1. Having Direct3D 11 (Advanced Layers) enagled => the issue won't occur.
2. A mozregression was made, and the results are:
   last bad build:
   build_date: 2019-08-26 10:54:36.303000
   changeset: c75d6a0539eb4b2c7961f0c9782fcb364198c6b2
   pushlog_url: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=98cbe150f012619bf9e8d44413fa33374e92dbc9&tochange=c75d6a0539eb4b2c7961f0c9782fcb364198c6b2
   and the message was:
   Narrowed inbound regression window from [98cbe150, c75d6a05] (3 builds) to [dcd9ee80, c75d6a05] (2 builds) (~1 steps left)
   2019-09-12T12:05:21: DEBUG : Starting merge handling...
   2019-09-12T12:05:21: DEBUG : Using url: https://hg.mozilla.org/mozilla-central/json-pushes?changeset=c75d6a0539eb4b2c7961f0c9782fcb364198c6b2&full=1
   2019-09-12T12:05:22: DEBUG : Found commit message:
   Merge autoland to mozilla-central. a=merge

The regression window that you gave is mozilla-central merge. Are you sure it's correct? Can you double check that you get the same window?

Hi @Jeff Muizelaar, sure, I've made another mozregression (by selecting the period 2019-08-05 to 2019-08-31)=> the same results are displayed as presented above-I'll attach a screenshot to see it. I've selected those dates because in 2nd of September the nightly was 71.0a1 anf beta 70.0b3 (on which the issue occurs too. So, from 26th of August nightly 70.0a1 which is now in 12th of September beta 70.0b5 the issue will occur.

I can reproduce this on my Windows 10 machine.

I ran mozregression and got a more specific regression range for bug 1574592. Could you take a look Markus?

This is already fixed on Nightly and the fix needs to be uplifted to Beta.