Okay, this is a behaviour I've observed in the past with other UI elements, so it's likely I just cannot wrap my head around how the whole anchoring system works in Godot. But this makes for a good example.
See image, I have a VBoxContainer with a menu bar and a color rect. The color rect is set to expand to take up the remaining space in the container. The menu itself has two buttons (and both have text and a size), though the height of the menu bar remains at 0 .
I know I can use the minSize settings to force a height on to menu bar even though its width/height are controlled by the parent container, but it seems counter intuitive. How can I get the menu bar to expand dynamically in order to fit its children? That is the default behaviour of parent UI elements that I'm used to, i.e. in web development, so I always fall into this line of thinking.