TreeViewer: auto expansion for elements with single children #2578

Job	Run time
check-freeze-period / verify-freeze-period	1s
build / Upload Event File	2s
check-merge-commits / Block Merge Commits	9s
build / Verify Linux	24m 51s
build / Verify Windows	32m 35s
build / Verify MacOS	31m 37s
	1h 29m 15s

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