So, there's this idea, which you already know: Define the layout of your UI by creating a tree of panels. The leaf nodes on the tree are what we used to call 'controls' way back in the day-- the things that the user interacts with, radio buttons and listboxes and such. The internal nodes are mostly concerned with layout; this kind of panel stacks its child panels vertically, that kind puts its children into a grid, etc.

It's COMMON. Most of the UI-generating systems I've seen in the past twenty years are implementations of this, and the ones that aren't borrow from it.

What's the word for this idea?