to reproduce view the attached example.

if you do things separately they work and render fine:
frozen tree with scrolling -> works fine
hiding/showing columns works fine (using the "+/-" buttons) -> aux headers resize properly and columns dis-/re-appear

when combining both (collapse e.g. "Q1") and then scroll the grid horizontally
-> the hidden column headers suddenly consume some width and shift the remaining headers to the right breaking the layout

problem also recorded here: http://screencast.com/t/MBZ1HzONcXnh