VSplitView layout explanation?

Can anyone here explain how the VSplitView container is laid out by SwiftUI on macOS? I had a view that was using a VStack container. When I replaced it with a VSplitView container, the view ended up present in a narrow vertical strip inside its containing view.

Here's what it looks like with a VStack:

Here's what it looks like with a VSplitView:

More generally, are there any good sources for understanding the inner workings of SwiftUI components? I know about Chris Eidhof's book but it seems limited as to how individual components work.

VSplitView layout explanation?
 
 
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