NS (darwin): What is the utility of update_window?

NS (darwin): What is the utility of update_window?

[Keith David Bershatsky]

It is my understanding that 7946445962372c4255180af45cb7c857f1b0b5fa provides compatibility with MacOS (Mojave) by restricting Emacs in such a way that nothing is written to the glass until the OS normally draws rectangles during the redisplay cycle -- forcing everything to happen with drawRect (expose_frame).

Other than marking dirty rectangles during update_window, does anything else useful occur during update_window on the NS port of Emacs?  If so, what else is accomplished during update_window?

BACKGROUND:  Based upon my observations, it would appear that Emacs does extra work on the NS port that is not needed on the W32 and X11 ports.  Whereas the W32 and X11 ports can update the glass during update_window, the NS port has to wait until a later point in time to accomplish almost same thing (but not as well).

NS (darwin): What is the utility of update_window?

[Keith David Bershatsky]

PS:  I am aware that the desired_matrix is compared to the current_matrix, and that the current_matrix is brought up to date during update_window....  The question in the initial post was essentially geared towards ascertaining whether anything is actually changed on the glass during update_window on the NS port of Emacs.