It's well explained here -> https://bbs.archlinux.org/viewtopic.php?pid=1500613#p1500613 tl;dr: Using XRender makes the plasma panel transparent, OpenGL doesn't. What's the intended behaviour here? IMO it should be transparent regardless of the backend. Reproducible: Always
You do realize that this is upstream, right? All the relevant information for a bug should be placed here, not in a forum thread on archlinux. Please do add the proper description here along with the screenshots. For what it's worth, i also find it a bit weird that the taskbar is more transparent with XRender then it is witl OpenGL. I also think this isn't breeze specific. You just happen to see it in breeze. I'm guessing it's kwin.
It's because the Background Contrast effect is available in OpenGL, but not XRender. The OpenGL version is the intended design.
Created attachment 90966 [details] Transparency with OpenGL
Created attachment 90967 [details] Transparency with XRender
@Mark Dunno how to edit the description. I added the screenshots as an attachment and moved the bug to KWin. @Eike Hein Well ok... but it looks better with more transparency. Especially for dark backgrounds.
Note that you can go to Desktop Effects and disable the Background Contrast effect if you prefer the look without it. Its introduction has been very well received, though. It's unlikely the effect will be ported to XRender, I think. It might be possible to make the asset used in XRender mode look similar to some degree, but then the user can't disable it anymore since it's baked into the theme. Overall I'm not sure there's much to do here, although consistent behavior across all modes would be nice, of course.
intended design as explained.