While the effort for creating an overlay that's painted on top of the original textbox is well intended, a simpler approach would be to open a fixed size window

While the effort for creating an overlay that's painted on top of the original textbox is well intended, a simpler approach would be to open a fixed size window on a fixed position that the user can configure. This way you could set it to your preference and it would work for all kinds of box sizes. Or maybe give the option to only fall back to this fixed size, fixed position textbox if the box dimensions are smaller than a predefined size.