STR:

1. Set the OS scaling to 250% (in Windows 10 this is "Change the size of text, apps, and other items")
2. Start Firefox
3. Change the scaling to 300%
4. Press and hold Ctrl+Tab

The tab thumbnails are the wrong size, overlapping.

This generally happens for me when I move the window between two monitors with different scaling. Sometimes the whole popup appears at the top of the screen instead of centered. Restarting Firefox corrects the issue.

I suspect that this is a Win32 widgets issue, but it might be more general.

The priority flag is not set for this bug.
:aswan, could you have a look please?

For more information, please visit auto_nag documentation.

https://hg.mozilla.org/mozilla-central/rev/844bddb7d0f4

I didn't managed to reproduce the issue following the STR from the first comment, I tried to reproduce the issue on Win10 on both x32 and x64 builds. If you could please verify that the issue is fixed using this link: https://archive.mozilla.org/pub/devedition/candidates/70.0b11-candidates/build1/.

Sorry, the STR was incomplete. A better version is:

1. Set scaling to 250%
2. Start Firefox
3. Open many tabs, enough that they won't all fit in the ctrl-tab thumbnail view (this has to be done before changing the scaling, I think)
4. Ctrl-tab, the first few thumbnails will appear properly spaced
5. Change scaling to 300%
6. Ctrl-tab, the thumbnails overlap

Using this I was able to reproduce on 69.0.2, and was not able to reproduce on the dev edition build linked in comment 5. Thanks!

Thank you for the help. I have verifyed that the issue isn't reproducing on the latest beta to.