MainWindow: Mark inconsistent and non-deterministic aspects of closin… #639

Job	Run time
build-linux (ubuntu-22.04, 3.8)	2m 58s
build-windows (windows-2019, 3.8)	8m 18s
build-macos (macos-latest, 3.8)	5m 0s
build-windows (windows-2019, 3.9)	8m 55s
build-macos (macos-latest, 3.9)	7m 43s
build-windows (windows-latest, 3.8)	8m 10s
build-linux (ubuntu-22.04, 3.9)	2m 29s
build-windows (windows-latest, 3.9)	12m 47s
	56m 20s

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