I took a screenshot (with Gnome screenshot app) of PureBrowser and it turned into what looks like a mix of QR code, a chess field and an 80s wallpaper.
By the way, this evening I observed the same "checkerboard" display corruption on a Kate (editor) popup dialog after taking a screenshot of it with Alt-Prt Scr on a Librem 15v3. That means it's not just an issue with Web browsers. This is the first I've seen this, and I could not get a screen capture of it or I would have included it.
I was completely up to date with PureOS this afternoon, but I see a few updates await me this evening.
Yesterday’s system update showed many GNOME packages upgraded to 3.30.0, but it did not fix this issue.
To clarify, the window remaining onscreen is what gets the display corruption. The screenshots themselves turn out fine.
Double-clicking the top border of the corrupted window to maximize the window will clear the display corruption. Double-clicking again to return the window to its previous size causes no issue.
I did the same and the problem still occurs, but I noticed that gnome-screenshot was not updated in the latest batch. (Now 9/15. Because of Florence, I did not login and update the past couple of days.) On my system, it is still 3.26.0-3.
This may be fixed with today's (9/16) update. gnome-screenshot itself was updated to 3.30.0 and appears to work for me.
UPDATE: I should have thought of this: ‘Grab the whole screen’ and ‘Select area to grab’ seem to work for me. ‘Grab the current window’ does not.
I also just got the 3.30.0 update for GNOME Screenshot, and 'Grab the current window' still shows the display corruption. As before, 'Grab the whole screen' and 'Select area to grab' do not have any problem. So, the bug is limited to grabbing a window.
By the way, I used both the GUI and the keyboard shortcuts, and there is no difference in results.
As of today's updates (2018-10-08), this issue persists and is still seen with a number of applications: PureBrowser, GNOME Web (Epiphany), Kate, and Okular. It seems to have been fixed for GNOME Files (Nautilus) and Thunar.
Thunar was updated today from 1.6.15 to 1.8.1 and while the screenshot issue disappeared, now it crashes immediately when clicking on any file. Crashing does not happen if logged in as GNOME on Xorg.