password field unusable if display configuration changes while on screen lock

I often lose the ability to type in my password on a locked screen (gnome3), if the display configuration changes.

Steps to reproduce:
* plug off external display
* lock screen in gnome (ctrl-alt-l)
* plug in external display

After these steps the password field cannot get focus or be used to type password in.
Sometimes also the buttons are without function, not clickable with mouse or reachable with keyboard shortcuts.

Plugging off the previously added display can sometimes help to recover to responsive elements, but often the password field stays unresponsive.
A workaround is then to use the "change user" button, and logging into the account. This usually works and circumvents a stalled gnome-screensaver.

Additionally, while testing for this bug report, I could use the desktop (which should stay locked) for quite some time (~20 seconds), before gnome-screensaver took over and kicked in again. This happened after plugging off an external display and the corresponding display settings change.

The latter could be a serious issue, related to bugs about resume reported elsewhere, e.g.
#1509711
since a display change can be easily forced.

Package: gnome-screensaver
Version: 3.6.1-7ubuntu4