I dont know if the apps choose their size and can override system preferences? I *always* use Firefox in Fullscreen and close it like that. But it always openes non-fullscreen. Weird thing, this does not apply for Thunderbird. Both are the Flatpaks from Flathub, I use Librewolf too, same effect. I cant say about RPM Firefox. Fedora 38 Librewolf based on recent Firefox, Flathub Flatpak
When you say fullscreen, do you actually mean fullscreen, or do you mean maximized?
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I mean maximized. I always click on their CSD "fullscreen" button and have it in fullscreen. But "macos ugly like" it always opens vertically fullscreen, horizontally with a bit of space. It never seems to remember my sizing. Is this something Firefox has to change or Plasma can enforce?
So my fix was to set the programs window properties to "vertically maximised" and "horizontally maximized". But in a former issue I already mentioned how extremely confusing the "set window properties" dialog is (hard to find, unclear whats given properties and whats wanted properties)
We could fix this in KWin by fixing Bug 437089, or you would report this in Firefox and they could fix it themselves by setting a real maximized state when they save their window state. *** This bug has been marked as a duplicate of bug 437089 ***
https://bugzilla.mozilla.org/show_bug.cgi?id=1850194