I got this pop-up recently and while it must be from some desktop application that uses Electron.js, it’s not apparent which.
An easy way to tell is to run:
This will change the mouse pointer (to a cross-hair pointer on my Gnome
3 desktop), as the
xprop invocation waits. Clicking with this on the target
window will make
xprop print a bunch of information about it, including the
name of the main application window and its pid:
... WM_CLASS(STRING) = "discord", "Discord" WM_ICON_NAME(STRING) = "Error" ...