Sometimes when the screen is locked and I press a key to unlock it, I see my entire desktop and open windows under the password dialog box. That's a security problem.
I believe this is a compositing / video memory issue, because of the following factors:
1.) It is intermittent, but occurs most when the screen is "busy" (lots of graphics-heavy windows open).
2.) I can almost always reproduce it by doing something video-intensive, such as x11perf.
3.) Sometimes everything will be visible except that one or two windows, or the "active" portions thereof, will be blacked out.
4.) I have seen other compositing / video memory glitches, such as the contents of a previously closed window appearing briefly in a newly opened window before it completes drawing (another security issue in its own right).
5.) The desktop is shown as it appeared when the screen was locked, and does not update while locked.
6.) I am unable to reproduce the issue with compositing ("desktop effects") disabled.
Note that the desktop is not dimmed; it is visible for as long as you want to stare at it, even after the password prompt has disappeared; and I am using dualhead and both screens are visible, so this is not a duplicate of #390989 or #385102 or #417722, though it is probably related. I have attached two screenshots (acquired via VNC) demonstrating the problem.
Reproducible: Didn't try
This bug has been originally reported in Launchpad: https://bugs.launchpad.net/ubuntu/+source/kdebase-workspace/+bug/474654
*** This bug has been marked as a duplicate of bug 246623 ***