I don't remember why, but suddenly my GNOME programs Peek GIF Recorder and Swell Foop puzzle game run with non-visible and abnormal titlebars on my KDE Plasma Desktop 5.12.3. It seems that this way GNOME Apps cannot show its top-left-corner menu and some other fail to draw its titlebar (we perceive it transparent). At this moment, I can only say that this problem occurs only if I choose Breeze for GTK3 theme. If I choose Emacs theme, it goes back to normal. A quick fix to this problem is by going to System Settings > Application Style > Window Decoration > GNOME Application Style > Select a GTK3 Theme > choose Emacs > OK.
Before:
(Not normal)
(Peek gif recorder: see how transparent the border is)
(How I am supposed to resize the window with this?)
(Swell Foop (puzzle game): see the unusual title bar and top-left menu cannot be opened)
After:
(Normal)
(Peek with normal titlebar and window border)
(Swell Foop (puzzle game): the menu can be opened again)
References
https://github.com/gnunn1/tilix/issues/1451
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