When GNOME screensaver starts to blank the screen there is a fading effect which takes a few seconds and is not interruptible by moving the mouse or pressing keys. This is very annoying. This should be interruptible by input actions, like the screensaver.
I can confirm this behaviour. It has been present since the switch to F13. In previous versions, the screensaver fade would deactivate if keys are pressed or if the mouse is moved. However, in F13 it seems that the screensaver will fully fade to lock no matter what you do.
*** This bug has been marked as a duplicate of bug 620264 ***
These 4 bugs seems to be related, or are the same one.
bug612620 Fade-out to screensaver cannot be interrupted
bug615786 As screensaver activates and screen dims cannot intervene
bug620186 The fading of the screen when GNOME screensaver start is
bug620264 gnome-screensaver activates whether or not activity occurs
once dimming starts