Under some GNU/Linux environments, the appearance of window-decorations can differ drastically between Qt5-based and Qt6-based builds of Gridsync -- more specifically, the Qt5-based builds appear to use the underlying "native" (GTK?) decorations, but the Qt6-based builds do not:

It would arguably be preferable if Gridsync/Qt6 could use the user's preferred/existing window-decorations over whatever Qt6 is currently doing.