Basically, what I do is:
If there are one or zero columns, hide the horizontalScrollBar until it is needed. (it has worked in the past, but in another file).
Apply the same strategy with files.
Additional stuff:
Moved the common calculus of itemSize outside of the if then else.
Testing
Krunner config does not loop (neither kgetnewstuff from kstars). I can not test with amarok (I've hit by an amarok start bug).
Please, test with other programs.