I orginally thought it was some instability with the graphics stack I was using (was using pontostroy’s drm-kernel and x11 repos), but even without those repos, GNOME seems to randomly lock-up.
The mouse pointer is still responsive. Everything else (animations on open apps, cursor changes when on edge of windows and text, Activities drop-down, etc) is frozen though. I can drop to TTY (is that the right term?) though with Ctrl + Alt + F#.
The GPU doesn’t restart when the freeze happens too, so I’m not entirely certain if it’s driver-related.
I can seemingly reproduce this under KDE 4 also, but XFCE and KDE Plasma 5 seem to be unaffected.
I do not know why you did not start this in the Tumbleweed forum, because that is of course the place where your fellow Tumbleweed userss are looking eager at every new Tumbleweed problem to provide help.
I know it’s my fault for not checking, but I didn’t even know there was a Tumbleweed-specific forum until now (would have gladly posted it there had I known).
As for some more feedback on the issue; stability seems fine on Fedora 21 (Mesa 10.4.3, LLVM 3.5, kernel 3.18.6). Not entirely sure which packages/versions differ between the distros though exactly.