Basically what the summary says. Starting Title crashes compositing. On one screenshot you can see konsole with its shadow, transparency and rounded window decorations. On the other (with mpv window), all those things crashed. I don't think that the whole kwin crashed because the window managing still works. But the compositing goes down.

I haven't managed to find any more applications that does that. Only mpv so far. It wasn't an issue on 5.5.

I can go to System Settings -> Compositor and change some setting there and back again to start the compositing back up.

/var/log/Xorg.0.log doesn't show anything.

Reproducible: Always

Steps to Reproduce:
1.start mpv
2.
3.

Actual Results:  
kwin compositing crashed

Expected Results:  
kwin compositing doesn't crash

$ pacman -Qs nvidia-340
local/lib32-nvidia-340xx-libgl 340.96-1
    NVIDIA drivers libraries symlinks (32-bit)
local/lib32-nvidia-340xx-utils 340.96-1
    NVIDIA drivers utilities (32-bit)
local/lib32-opencl-nvidia-340xx 340.96-1
    OpenCL implemention for NVIDIA (32-bit)
local/nvidia-340xx 340.96-8
    NVIDIA drivers for linux, 340xx legacy branch
local/nvidia-340xx-libgl 340.96-1
    NVIDIA drivers libraries symlinks
local/nvidia-340xx-utils 340.96-1
    NVIDIA drivers utilities
local/opencl-nvidia-340xx 340.96-1
    OpenCL implemention for NVIDIA
$ pacman -Qs kwin      
local/kwin 5.6.1-1 (plasma)
    KDE Window manager
local/kwindowsystem 5.20.0-1 (kf5)
    Access to the windowing system
$ uname -roms
Linux 4.4.5-1-ARCH x86_64 GNU/Linux