improve flutter floating window handling #5

Job	Run time
build (ubuntu-24.04, ci-dev-qt5, 5.15)	7m 4s
build (ubuntu-24.04, ci-release-qt5, 5.15)	3m 4s
build (ubuntu-24.04, ci-qtwidgets-qt5, 5.15)	2m 37s
build (ubuntu-24.04, ci-qtquick-qt5, 5.15)	2m 23s
build (windows-latest, ci-dev-qt5, 5.15)	27m 33s
build (windows-latest, ci-release-qt5, 5.15)	3m 32s
build (windows-latest, ci-qtwidgets-qt5, 5.15)	2m 54s
build (windows-latest, ci-qtquick-qt5, 5.15)	2m 26s
build (macos-13, ci-dev-qt5, 5.15)	25m 19s
build (macos-13, ci-release-qt5, 5.15)	5m 14s
build (macos-13, ci-qtwidgets-qt5, 5.15)	1m 52s
build (macos-13, ci-qtquick-qt5, 5.15)	3m 33s
	1h 27m 31s

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