How to nail a button to the position of the Node2D
If the button is a direct child of the Node2D, it already is! The
position-property is always relative to the parent-node, it it's set to (0, 0) the top-left corner will be the position of the parent.
The button is always too big
You can set the
rect_size-property to precisely the value you want, let's say (200, 20). If you want the button to be centered on the position of it's parent-node, set the
rect_position accordingly, i.e. in this case to (-100, 10).
and does not behave correctly
Which behavior do you expect? And what's happening instead?