I decided to mail about this as well... granted I've had no success bringing it to the attention of the Tumbleweed crew in two weeks, and my system crashes daily because of it. Below is my bug report and a short description of what's happening, logs are attached to the report in cause. Please let me know what you think... and if the source is identified and such is possible, please consider rolling back the change that introduces this problem! Thank you.
Approximately once per day, the system experiences a random reoccurring crash: All windows and buttons freeze in place and become unusable, while only the mouse cursor remains possible to move around. After a few seconds, the monitor starts turning itself on and off several times (standby mode). Finally I either find myself back to the login manager, or the monitor permanently stays off... which of the two is completely random. If I manage to kill X11 in time (control + alt + backspace) and try logging back in, desktop effects will sometimes not work although they report no error as to why, whereas at other times the image just freezes again and I'm forced to restart anyway.
Today I discovered an important detail: The crash is not limited to KDE desktop effects, unlike most crashes of this sort and what I initially suspected to be the case; I had compositing disabled for several hours (alt + shift + F12), yet the exact same crash occurred! Although the freeze is rare and probabilistic so it's impossible to reproduce on demand, the trigger seems to be some sort of window activity, such as a system tray notification or window popping up... also starting up certain games may cause an immediate system freeze.
My OS is Linux openSUSE Tumbleweed, running on top of KDE / Plasma 5. I use the free video drivers and default system packages of this distribution. My card is a Radeon R7 370, GCN 1.0 on RadeonSI and the radeon module (no amdgpu support yet). The issue survived through several kernel updates, indicating it's likely not part of the Linux kernel directly. Current versions or relevant packages (monitored at http://tumbleweed.boombatower.com ):
kernel: 4.10.3 mesa: 17.0.1 llvm: 3.9.1 xorg-x11-server: 1.19.2 xf86-video-ati: 7.8.0