QXcbScreen::grabWindow does not properly handle windows which have input class InputOnly. According to https://tronche.com/gui/x/xlib/window/create.html#InputClass, windows of InputOnly cannot be used as drawable.

Proposed solution: if the attributes reply has class InputOnly, fall back to capturing from the root window.

Attached is a xscope log that demonstrates the X11 communication. First there is one request that fails, followed by one that succeeds. The window ID is a foreign one (the application in question is a screenshot tool).