User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:69.0) Gecko/20100101 Firefox/69.0

Steps to reproduce:

Just browsing normally.

Actual results:

1. During any page rendering, the browser is slow and makes GRE Omni ParentBuildID process busy with up to 12-30% of all multi-core CPU resources, occupying up to 100% overall.
2. In case of actions such as resizing/maximizing/minimizing the renderer might randomply stop working at any point as seen in the attached screenshots.

Expected results:

Fast, low-resources-consuming rendering.

(In reply to User Dderss from comment #1)
Ran mozregression with an older copy of my main profile and was asked to start in safe mode:
Attached screencast looks simlar to comment 1, but it's bug 1489463 with a different appearance.
(Just in case of doubt: My also running main Nightly has a different theme. It's really a corruption of this window.)

Window restored maximized, but it's content then shrinked to the smaller size. Exceptional is that this time I was able to hover and use buttons and scrollbar on the position they were painted.
Usually the gap between real window size and window content size is transparent (attachment 9037655 [details]) or black (attachment 9039753 [details]) and element positions are within the real window regardless how smaller it's painted.

User Dderss, how easy is it for you to reproduce the problems that show up when resizing/minimizing etc? Can you make it happen on demand?

It is not hard to reproduce in my case, I just had to play around a bit making the window quite small after which making it bigger makes the context disappear no matter if I was gradually resizing it or maximizing it. After that, there are only two ways to continue working with the page: do it in a tiny window or reloading it.

This behaviour might be a separate issue from the issue of WebRender being slow/high-CPU usage, but I am not sure.

By the way, unrelated to this specific attachment: one of the previous attachments showing rendering dropped halfway might be derivative from just how slow the rendering is so in some cases it is simply interrupted/cut by the main FF process or something like that.