Hi there,

as result of Canonical's role-out of mesa 18.0.5 replacing 17.2.8 for Ubuntu Core 16.04 LTS, some users running KDE Neon User Edition and while being on Plasma version 5.13.2 to 5.13.4 + using Intel integrated graphics (especially Broadwell) experiencing loss of transparency/opacity in their Plasma Theme. 

relevant components:

experienced with Neon in KDE Plasma version 5.13.2, 5.13.3 and 5.13.4
Xorg 1.18.4
Mesa 18.0.5 using OpenGL 3.1

The experiences are exchanged and documented (with screenshots) on: https://plus.google.com/105447283901275565289/posts/dMAvFe3pwbg

I had a look into possible duplicates, especially into: https://bugs.kde.org/show_bug.cgi?id=392716 and further then related to https://bugs.freedesktop.org/show_bug.cgi?id=105871. 
But I am not sure, if this is to 100% the same issue, because disabling Compositor on startup doesn't change anything.
And I am also just a user, therefore not too deep into that stuff, but happy to help, to provide more info and to test, wherever possible.

For sure is, that the transparency towards KWin (Window Manager), configurable via Desktop Effects (e.g. window menus, dialogues etc.) is not affected and working properly. 
Only plasma theme related details, like control panel, application menu and Konsole is experiencing this loss of transparency.

Other reported effects, like in Kubuntu 16.04 with Firefox Menus, e.g. https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/1780664 
or https://bugs.freedesktop.org/show_bug.cgi?id=103699 
are not experienced in Neon. 

One user (GIMP PRO) in the given Google+ thread, who experienced the transparency loss as well, in Plasma version 5.13.2 and 5.13.3 recently commented that it is restored on his system with the Neon update to version 5.13.4. 
It is not yet confirmed, what Intel hardware he's using.
But Broadwell is definitely still affected.

I hope, that this helps and that a solution can be found.

Have much thanks in advance,
Andreas