Hi Pascal and Mark,
first, I've noticed the size changing behaviour in TextMorph too.
I wanted a text which would start editing on single click and apply its changes on Enter key or on loss of focus.
In my (years old) application I subclassed StringButtonMorph (which looks like a text not like a Button) and changed mouseDown: (read the comments!).
pvnd> starting: see EinMalEins>>step,
My class comment suggests I was not happy with stepping.
If you need help how to plug this into a model, come back.