When I lock the screen in Gnome Shell, a shield screen appears when I
hit any key (including e.g. Shift). This is good.
However, when the screen saver/monitor power save kicks in, but the
lock is set to a higher timeout, e.g. 30 minutes, no shield screen
appears after hitting a key. Instead, whatever key you typed is sent
to the application that has focus, but you can't know what application
that is ahead of time because the screen is in power save. I find
myself accidentally typing my password into random windows because I
assume the screen is fully "locked" but it is just in power save mode.
I.e. it is hard to distinguish between these two cases.
Proposal: invoke the shield screen even when the screen isn't fully
"locked" yet, just in power save, to prevent keystrokes from being
erroneously sent to applications when waking up the screen.