screensaver locks up when running with efifb (nomodeset)
This is from the openSUSE bug report: https://bugzilla.opensuse.org/show_bug.cgi?id=1210562
When the system is with efifb (i.e. without DRM) and XFCE screen lock is invoked, it doesn't go to the lock-screen (no login dialog) but just hangs up. Namely, the keyboard input and the mouse focus is no longer taken while the mouse cursor can still move. You can change to other VT via ctrl-alt-Fx, and kill xfce4-screensaver program to recover.
As a simple reproducer: just boot your system with "nomodeset" boot option, start XFCE and and do locking screen. Note that the issue isn't reproducible on the latest openSUSE Tumbleweed, just because it uses simpledrm for the efifb.
The cause of the hang up is the hackish check of the existence of a real monitor in src/gs-manager.c. With efifb (i.e. without DRM modesetting), no monitor is reported, and it's the single screen. As a result, it continues (or aborts?) without handling things fully, leaving the whole screen modal.
A workaround patch is found in the bugzilla: https://bugzilla.suse.com/attachment.cgi?id=866386
But I'm not quite sure whether it's 100% correct, as I didn't quite follow the reason of this check (introduced in commit c44e5435).