Why is margin_bottom 8?
After you change, the circle control will be perfectly circular. But value_slider's height is still 8 more than circle.
So I want to know why should margin_bottom keep 8?
New layer:

Without the margin, the shape position is asymmetrical vertically:

Although it seems to happen only in the editor theme. Default theme has opposite issue...

EDIT:
Nevermind, it's just the panel has a huge margin. That's a separate issue I think.

EDIT2:
I removed the MarginContainers completely.

Thanks!