I discontinued using the “Oxygen” window decorations in Plasma because I’ve noticed that they cause the minimize, maximize, restore, and close buttons to be disabled on any GUI app after returning from full screen. This can be rectified by quadruple-clicking the app’s Title bar, but I consider that to be “not worth the hassle”, so I switched to “Breeze”. I’ve seen this on several computers running different CPUs, different graphics (Radeon, Nvidia, Intel) and different window systems (X11, Wayland).
Steps to recreate bug: tap F11 key twice in any GUI app (if “Oxygen” window decorations are being used).
Workaround: Use “Breeze” instead of “Oxygen”.
Should I file a bug? Wait, let me check… no, it’s already a known bug (500976) since March. No action in 7 months, so I figure it must be considered either “hard to fix” or “low priority”. Maybe by 2026 or 2027.
I’d fix it myself and submit to gitlab but something tells me I’d have to do a lot of learning first.
Mod edit (scotty65): added a link to the KDE bug report
… I’d fix many things myself - if I just knew how because I had learned it … 
Possibly, since Oxygen is not required for Plasma. I haven’t had it on my system since the middle of last year, and today I also removed the oxygen-icons package, which was only an optional dependency for backintime-qt:
paclog --grep=" oxygen"
...
[2024-05-08T17:57:11+1000] [ALPM] installed oxygen (6.0.4-1)
[2024-05-08T17:57:11+1000] [ALPM] installed oxygen5 (6.0.4-1)
[2024-05-08T17:57:12+1000] [ALPM] upgraded oxygen-icons (1:5.115.0-1 -> 1:6.0.0-2)
[2024-05-08T17:57:12+1000] [ALPM] upgraded oxygen-sounds (5.27.11-1 -> 6.0.4-1)
[2024-05-19T10:27:20+1000] [ALPM] removed oxygen-sounds (6.0.4-1)
[2024-05-23T11:58:50+1000] [ALPM] upgraded oxygen (6.0.4-1 -> 6.0.5-0)
[2024-05-23T11:58:50+1000] [ALPM] upgraded oxygen5 (6.0.4-1 -> 6.0.5-0)
[2024-05-27T19:25:52+1000] [ALPM] upgraded oxygen (6.0.5-0 -> 6.0.5-1)
[2024-05-27T19:25:52+1000] [ALPM] upgraded oxygen5 (6.0.5-0 -> 6.0.5-1)
[2024-06-20T13:25:56+1000] [ALPM] removed oxygen5 (6.0.5-1)
[2024-07-14T11:39:51+1000] [ALPM] removed oxygen (6.0.5-1)
[2024-10-24T09:05:04+1100] [ALPM] upgraded oxygen-icons (1:6.0.0-2 -> 1:6.1.0-2)
[2025-11-01T10:23:46+1100] [ALPM] removed oxygen-icons (1:6.1.0-2)
As I recall Oxygen - it was a Plasma 4/5 theme - which was replaced by Breeze - and scheduled to be removed over time - but I may be wrong.
Oxygen was introduced in Plasma 4, and was the default there. Breeze also already goes back to that time, but at that stage it was still somewhat experimental, until Plasma 5, where Breeze then became the default.
Is Oxygen still an option in a new install? I typically just take the default without any further thought.
Edit:- I see, no, it’s not.
I can’t say I noticed it’s demise, but it’s still in the repo’s.
There are different theming aspects with that name, and the package name may actually be confusing.
As I understand it, there is a new Oxygen application style for Plasma 6 as an upstream-supplied alternative to Breeze, but I don’t know whether they’ve also rewritten the window decorations and/or the Plasma style. So I don’t know which styling aspect the package in the repo is supposed to represent. 
With apologies to those who still happen to like the old Oxygen styles, anything would be an improvement.
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