> IIUC the problem I see can be described more easily as follows.
 > Evaluate in sequence the following expressions with emacs -Q:
 >
 > (modify-frame-parameters nil '((menu-bar-lines . 0) (height . 30)))
 > (modify-frame-parameters nil '((menu-bar-lines . 1) (height . 40)))
 > (frame-parameter nil 'height)
 >
 > Here the height reported by the last form is 41.

Actually, the immediate cause of this problem goes as follows: When you
specify both addition of the menu bar and the new frame height,
‘modify-frame-parameters’ will first issue a request to add the menu bar
and then a request to calculate the new frame height as specified.

However, Emacs does not immediately add the menu bar but delay this
until prepare_menu_bars is run by the display engine.  Till then, only
the value stored in the external_menu_bar slot of the frame tells
whether the frame shall have a menu bar or not.

As a consequence, the request to set the frame height will be delivered
first to the toolkit.  The calculation of the new frame height on
Windows is done by calling the function AdjustWindowRect.  The third
argument of that function has to specify whether a menu bar should be
included in the calculation or not.  Currently, this value is set to
whatever the external_menu_bar slot stores.  But if the frame does not
have a menu bar so far but should get one in the future, the value
stored there is inherently wrong for calculating the size of the frame
height via AdjustWindowRect.

I attach a patch against Emasc-25 which fixes this bug here.  I also
added a remark to the doc-string of ‘modify-frame-parameters’ in the
sense that if you specify several parameters which might affect the size
of the frame, Emacs cannot always guarantee for every toolkit that the
final size will meet the expectations of the caller.  Inherently this
means, that the application programmer has to experiment with this
function on each and every toolkit involved.

I leave it to Eli to decide whether the patch should go into Emacs-25 or
master/trunk.

Thanks for your attention, martin