[SOLVED] Plasma X11 transparency broken

[PureTryOut]

Since an update from like a month ago (not sure what was updated at the time), transparency has been broken for me using Plasma on X11 (it works fine on Wayland). Everything that's supposed to show any opacity shows black instead. For example the main panel and the Latte dock background.

How my Latte dock shows (which has background set to transparent):
https://i.imgur.com/mX7IqHh.png

This happens using Qt 5.9.4, Plasma 5.11.5, and Mesa 18.0.0_rc4 on Intel Integrated Graphics 530.

[asturm]

Make sure compositor is enabled in systemsettings, remove ~/.cache, downgrade to mesa-17, in that order.
_________________
backend.cpp:92:2: warning: #warning TODO - this error message is about as useful as a cooling unit in the arctic

[PureTryOut]

Downgrading Mesa did the job, thanks. A shame, but I'm glad it's resolved.