The composition string is displayed on top of the edit control, and doesn't move the underlying text. This is a little bit similar to Windows, but it still looks like it manages to shift some text there, though it's not clear how and it's also not very accurate either, so hopefully also something that could be improved later.
Actually I misremembered, and MS Japanese IME doesn't move the edit text while compositing. It overlaps the edit control just like it's done here.