gcr-prompter is used by gpg to enter the passphrase, and though the
prompter has the focus, the keys are not taken into account unless
the mouse pointer is over the prompter window.

Also, it seems that all the keys are trapped, which may be expected,
but in such a case, they should be taken into account by the prompter
(just like Xterm's "Secure Keyboard" function, which works perfectly).

Note: my window manager if fvwm.