I can still reproduce this bug on Solaris SPARC with 7.0.86747.
While stopped in the debugger when the contract was
violated, I got some extra information:

   positions_and_sizes_try.item.pos = 368
   l_tool_bars.at(positions_and_sizes_try.index).size = 599
   sum of the above two expressions = 967
   tool_bar_row.size = 966

Since 967 <= 966 is False, the check assertion was violated.

Note also that reproducing the problem is simpler than the steps in the 
original report.  Just open a project with standard default layout,
resize the window a bit narrower so that the Stop Execution button is the
last one visible in the top tool bar.  Then undock the Standard buttons tool
bar by moving to the left a bit, but keep mouse button held down.  
Move it back to its original position still keeping mouse button down.
Check assertion is violated before mouse button is released.