bug#2530: 23/NS: redraws according to mouse-face are slow

[David Reitter <david.reitter@gmail.com>]

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On Mar 4, 2009, at 4:29 PM, Adrian Robert wrote:

>> I find that the redisplay of overlays that happens when the mouse is
>> moved into an overlay with a mouse-face property are much slower in
>> Emacs 23 (NS, under Cocoa/OS X).  It is pretty much a nasty animation
>> - every layer is redrawn from left to right, it seems, and every step
>> is visible.  It seems that background is drawn first, and then the
>> text over it.
>
> Yes, this has been an issue for years from Emacs on Aqua on and it  
> completely baffles me.  The NS code for handling mouse face is  
> identical to other platforms as far as I can tell, so I don't know  
> why the issue occurs only here.  And the animation is far slower  
> than any code on the NS side could be taking.  It must be a bug  
> somewhere on the core display side that is exposed because (guessing  
> here) the event loop under NS is done slightly differently.

Does anyone have an idea how to fix issue 2530?  I think this slowness  
is quite painful.  In my case, it is the tabbar.el variant that I'm  
using that causes this - I'm using several overlays (for a tab-close  
button, for instance) that get redrawn one by one.  I would imagine  
that this will annoy users in other use cases as well.

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