> OK, but this means you can compute the width of the tool bar as you do
 > for any other window, and then add to that the width of the fringes.
 > Since on Windows the tool bar is just a special window which doesn't
 > have fringes, this should be accurate, no?

It's not related to fringes.  I forgot to subtract the width of the
external borders.  Does the attached patch (mostly fixing indentation
hell) give the correct result?

martin