My example was one of the first things that I made with Anvil and was taking advantage of… the lack of control on margin and padding. You can see that the children elements in my tree have a nice indent on the left and a not so welcome one on the right. That’s the result of whatever was in the default css when I made that app.
Perhaps the default css in Anvil have changed and the margin/padding for that element on a new app created today is zero.
I tried to clone your app, but I wasn’t able to see the tree. Perhaps there was too much stripping down.
You can try to play with the css, perhaps creating a role that adds some margin on the left.
I think that using spacers should work too. You can try by holding the ctrl button while dragging the spacer size. This should give you finer control on its width. Or, as a last resort, you could try setting its width from the code.