If nobody else reports on having debugged this and tells what the
cause is, would you please debug it?
The cause is the toolbar. Since I never use it I never noticed
the bug before.
I can reproduce the bug with the single-line .emacs:
(add-to-list 'default-frame-alist '(height . 37))
I can't reproduce it with the two-liner:
(add-to-list 'default-frame-alist '(height . 37))
(tool-bar-mode nil)
I faintly recall some discussion about this on emacs-devel.
Maybe the bug has been already fixed on the trunk and people
can't reproduce it now.