When enabled the first starting position of any application window shall be in the center of the current screen. NO LONGER UPDATED! If you have 3.14 or higher gnome-shell, then using dconf-editor go to /org/gnome/mutter/, and check 'center-new-windows'.
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Couple of comments again: - let x = monitor.x//0; => now missing semi-colon at the end of statement - /* 0 = Normal Window, 3 = (non-modal) dialog */ that comment would not be necessary if you used Meta.WindowType.NORMAL and Meta.WindowType.DIALOG instead of 0/3 - in the metadata, "3.12" already covers all of 3.12.0, 3.12.1, 3.12.2, 3.12.3, ...
Place Windows In Center