I`ve ran into a rather annoying bug today, if you want to use either the _height or textHeight property of a textfield using device fonts and set to autoSize, it`ll add a phantom line, so will report the heights as if there was an extra line in the textfield.
The fix for it is quite random as well, you have to ADD a “\n” (=new line) at the end of your text and whooops, the phantom line is gone.