> While we're looking at this area, here's a suggestion. Can we 
> get rid of the resizing of every morph to be 9999 wide?

The reason for this is mostly to avoid (potentially expensive) resizing of
StringMorphs when you resize a PLM. In other words, with your changes (which
I haven't tried) it is likely that you have to click exactly on the string
and not "to the right" before the string ends. While I don't consider the
current solution to be very nice, the UI problems that your solution
introduces seem much worse to me. 

Cheers,
  - Andreas