As an addition for another (quite specific) case:
If you want something like the notification system in Civ 5 or similar games (widgets are filling up the vertical box from the bottom, with the newest one on the bottom, if there are too many, scroll), you can use this setup:
Your Hud Canvas Panel
->Canvas Panel anchored to your desired location (my example left side middle of screen) [Size to Content true]
->SizeBox also anchored to the same location [Size To Content True, width override and height override ticked, and set to your value]
->ScrollBox aligned to the left and bottom (so the newest notification is at the bottom)
(->Vertical Box here, but you don’t actually need it)
->Your Widgets
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