Disclaimer: I have no idea what I'm doing, feel free to correct me.
When 'Window scaling' is set to 2x, two (apparently foreseen: https://bugzilla.xfce.org/show_bug.cgi?id=14566#c5) bugs manifest themselves:
1) drop-down menus are restricted to the top-left quadrant of the screen
2) maximized windows are restricted to the top-left quadrant of the screen
I've looked at https://git.xfce.org/xfce/xfwm4/commit/?id=07773e608413a370932c5f2d59ed0b656240970a and inferred that gdk_monitor_get_geometry returns scaled dimensions/coordinates that should be scaled back by gdk_monitor_get_scale_factor, but note that it's purely a speculation, especially for the coordinates part. Applying that multiplier to the remaining invocations of gdk_monitor_get_geometry seem to solve the problem for me.
Unfortunately, I cannot test the result with multiple monitors right now, but at least the single-monitor scenario looks good to me.
Created attachment 8284
My attempt at scaling gdk_monitor_get_geometry results
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