Don’t know if this is caused by Manage popup windows in the GUI daemon · Issue #6931 · QubesOS/qubes-issues · GitHub but when running in multi-monitor, right-clicking in Firefox that’s not on the primary screen opens the popup on the border of the primary screen and not where I actually clicked. Is that a known issue? Can’t seem to find it mentioned anywhere.
I had same issues. A restart of a qube having this issue after the monitor connection and set up helped me a bit. It worked better when the monitor was connected before the system boot. But I guess the last method has an additional security risk of exposing the dom0 to the monitor (my had several usb ports).
The way I resolved the issue was by setting one of the external monitors as “primary” in dom0’s xfce settings. I dont know what it does, but now everything seems to work. The popups appear correctly and also some other badness is gone (like some windows not registering clicks on some edges of some monitors)