Since the 4.11 release we got a bunch of bug reports indicating that OpenGL Compositing broke with the legacy catalyst driver. If you run into this issue: switch to the free radeon driver. If you do not want to do so export the following environment variables:
KWIN_DIRECT_GL=1 KWIN_COMPOSE=O2

And it's possible that you get rendering issues with the normal Catalyst driver. Again: switching to the free driver fixes the issues.
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