I use the Panel Transparency Button on my desktop. You toggle it in edit mode and then it's just hidden in normal use.
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Plasma 6 Bugs
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I tried that. Unfortunately it doesn't play nice with adaptive transparency and makes the panel transparent all the time, so even when a window is maximised its still transparent. 😞
I think I found out how to do what you want. In the System Settings app, you will go to "Workspace Behaviour" (right under Appearance), then in the submenu choose "Desktop Effects", and either scroll down (number 2 in "Appearance") or search for "Blur". To the right there's a configure button that you press.
In the popup you can then configure the Blur Strength and I suggest you put it all the way to Light (to the left). Noise strength i have no idea what does lol.
Hope this helps, and if you need pictures I can do that as well
Edit: This will also change other places with blur.
And also remember to change the opacity settings of the panel (In edit mode, edit the panel, to the right "more options", and then you can choose from there)
Thanks for helping. I've already tried putting the blur all the way down (and fiddling with the noise, no idea what it does either) - the screenshots in my post are both with blur at 0 :( Also means my logout screen isn't blurred properly and the icons are harder to see over the background.
My panel opacity is set to "Adaptive" so that it turns opaque with maximised windows. Even if I set it to be "Translucent", it's looks the same as it is in my screenshots.
You might be able to pull this off with kvantum, although I don't know how.