Automatically calculated content size of ScrollablePanel was wrong wh… #582

Job	Run time
linux	4m 46s
linux-per-backend	4m 18s
linux-oldest	3m 10s
linux-latest-dev	4m 20s
windows-clang	6m 58s
windows-oldest	6m 12s
windows-sfml-sdl	2m 11s
windows-static-mt	5m 33s
android-sdl	4m 36s
android-sfml-graphics	2m 0s
ios-sdl	4m 2s
ios-sfml-graphics	1m 37s
macos	6m 52s
nightly-build-windows-visual-studio	10s
	56m 45s

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