To replicate: make sure that a terminal window (for example) has the input focus then place the pointer over the backdrop, invoke logout, cancel logout (via the keyboard).

Note that the same window (according to the frame and, possibly, translucency) appears to have the input focus. Now (without moving the pointer) try typing something.

Move the pointer into the window without crossing other windows (NOTE: requires focus-follows-mouse), type something (it appears), move the pointer back out, type something (it doesn't appear).

You need to click in the window or give a different window the input focus for things to return to normal.

Solution: trace logs show that, when the logout window is opened, grab and ungrab notifications are being sent. The interesting one is a grab notification with detail == NotifyInferior; just allowing this in addition to NotifyDetailNone is enough to fix this.