Hi!

I can confirm that I see the the problem. However, "Automatically hide and show the Dock" option in the OS X system preferences must be disabled.

The same problem occurs when Alt-clicking the green zoom icon.

This is probably caused by the following commit:

    http://git.savannah.gnu.org/cgit/emacs.git/commit/?h=emacs-25&id=ba24d35a3e82cdeba4be5bd794f7f48bbfa5498e

It was intended to ensure that `toggle-frame-maximized' used the full height of the screen -- earlier it left a few pixels at the bottom even when the dock was hidden. However, it clearly didn't take inte account the case when the dock was visible.

    -- Anders