The menu bar flickers for me on Windows, too. It is not as disturbing as for Arash, though.
I have attached a picture composed of four consecutive (partial) frames from a screen recording. In the uppermost frame the Emacs window is just about to lose focus, and in the three lower frames the menu bar is being redrawn after losing focus. In the second frame only the word "File" is displayed, in the third frame the words "File Edit Options Buffers" and only the fourth frame shows all seven menu headers.
This result was obtained the way Arash described: I started Emacs with emacs -Q, C-x 5 2 and then switched between the two frames.
Flicker.png