bug#9366: Display geometry change hook

[martin rudalics]

> Yes, I know about WM_DISPLAYCHANGE (although we currently only handle
> the full-screen frames there).

Has anyone ever confirmed that it correctly handles the full-screen
frames?

> But the corresponding X feature
> requires the use of a special X module, and I don't know what happens
> on macOS.  So I thought a platform-independent method that always
> works, and can be implemented in just one place, is a better
> alternative.

But such a platform-independent method on X would boil down to using the
same special X module as the platform-dependent method.  That is, we
would try running XRANDR in either case ...  I don't know about macOS.

> Besides, adding one more special event comes with minor disadvantages
> of its own -- one more event to disregard in situations like
> while-no-input etc.

But at least on Windows we have to pay attention to such events anyway.

martin