bug#29050: 26.0; Change in when `window-configuration-change-hook' is run

[Drew Adams]

> In earlier Emacsen, changing the frame size was allowed to delete all
> windows but the selected one.  Nowadays, changing the frame size does
> not delete windows any more and so does no more change the window
> configuration proper but only the sizes of individual windows.
> 
> 'window-configuration-change-hook' is a quite expensive hook which is
> run for way too many functions in the windows area.  It's expensive
> because it's run for every single invocation of these functions and
> often even if nothing had changed at all.  I plan to run it exclusively
> for the following cases - window deletion and creation and displaying a
> different buffer in a window - which are all real changes.  Any window
> resizing is covered by ‘window-size-change-functions’ which is now run
> (almost) only when the size of a window really changed.

OK.