What one property input box holds while it is being edited: the text as typed, and what that text means. The TEXT is model state, as it is for every other input box in Issie - the Constant's value box, the component name box - which dispatch the unparsed string on each keystroke and render themselves from it. Keeping only the parsed value here left the box with no way to be set except by reaching past React into the DOM, which is what the offer to bind a property to an enclosing sheet's had to do. Only a Spec that is Ok ever reaches a component: text that will not parse, will not evaluate, or breaks a constraint stays here with its message, and the design is untouched until it is valid.