I don’t know why but when i try to get a window screenshot for firefox or gnome calculator for example, i get black window screenshot and in the same time, not for xterm for example …
Window screenshot is used in Fvwm for Pager …
I don’t know why.
It seem that Window ID is not good … Got with identify but bad for window screenshot …
I have tried other capture methods (scripting an x tool, may have been in the manpage?) and had a similar result, I am not sure about a sort of ‘builtin’ related to the Pager. I use the most recent fvwm from the github repo which I rebuild for myself as new commits land.
However, here is more of a workaround than a direct solution. I use flameshot (Releases · flameshot-org/flameshot · GitHub) which works perfectly well for any captures I desire, plus it includes a small number of editing tools before you save or use the screenshot. Qt as a runtime dependency may be a bit much if you don’t already need it for other things.