Xfwm4 running on Mac under XQuartz has a glitch that causes dialogs to open behind the window that opens them (their transient-for window I presume). Sending a raise command to the parent or dialog restores the proper stacking, but changing the dialog tab/page (e.g. in many Qt5/KF5 applications) will push the dialog back again.
I suspect a more or less subtle unexpected interaction between xfwm4's stacking algorithm and XQuartz peculiars.
Additionally I noticed that many of the dialogs in question have a modal nature which makes it impossible to select/raise the parent window on Linux; no such behaviour on Mac (and TBH that's fine with me).
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