X11 menu windows opened on the wrong monitor
After plugging an external monitor I tend to have all my windows moved to a single monitor & single workspace.
I have to move the windows manually back to each in their own workspace and right monitor.
For some applications like thunderbird (x11 app) the menu windows now appear on the wrong monitor.
Here a right click on the blue element of thunderbird :
Closing the application and reopening it fixes the issue. This is gnome-shell 3.38.3 & mutter 3.38.2