I cannot figure out how to do this. The problem if I bind to the label font size is that the label font size is not relative to the box height (font_size: root.size[0] * .045), so you have to use absolute heights in which case you run into issues with it overlapping on the slider like you had initially. If you use relative heights then the font becomes small very quickly.

This wasn't a problem earlier, because for the label, even if the text extends outside the box, it still looks the same. For text input, however, the text outside the box becomes invisible.

Do you want to give it a try to fix it?