For the record, I have reproduced this crash on Windows XP (using the 
intermediate release 69141).  It crashes in {SPECIAL}.overlapping_move
since assertions are not being monitored.  Again, in the final step you
need to make sure the outline box covers the entire Clusters window, not
just the left half of it, so that all of the tools (Features, Output, Diagram 
etc) would become tabs that are together with the Clusters tab, if it
did not crash.

Since this sequence does not involve moving anything fast and since the
problem is not dependent on timing (it crashes every time) or making the
main window really small, this really should be fixed before release, unless 
the fix is too complicated and risky at this late stage.