PR# 13189 Violation of `not_outside' of {SD_TOOL_BAR_ROW_POSITIONER}.on_pointer_motion

While running under workbench estudio, I got a violation of assertion
`not_outside' of {SD_TOOL_BAR_ROW_POSITIONER}.on_pointer_motion.
Not reproducible yet.  Trace attached.

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.

I found a reproducible sequence for this problem.

To reproduce:

Start workbench estudio and run estudio from within it.
Open existing project.
Undock Standard Buttons toolbar and dock at bottom of main window (all the way
   to the left).
Undock Project toolbar and dock at bottom of main window, to the right of
   Standard Buttons and in the same row.
Undock Standard Buttons toolbar and move it over left side of main window
   so that it docks there vertically but don't release mouse button.
Move Standard Buttons toolbar back down to dock it at bottom of main window.
Estudio crashes with violation of assertion `not_outside' of
   {SD_TOOL_BAR_ROW_POSITIONER}.on_pointer_motion.

Attachments for problem report #13189