Proposal for overflow painting order

I'd like to propose a change to the painting order of overflow objects.

In this example:

<div style="width:100px;height:100px;background-color:red;margin- 
bottom:-100px">This text paints on top of the overflow block.</div>
<div style="width:100px;height:100px;overflow:hidden;background- 
color:green;"></div>

The overflow block is allowed to interleave with the previous block.   
Although its background is on top of the previous block, the previous  
block's text paints on top of the overflow block's background.  This  
is correct according to the current painting order rules, which do not  
special case overflow.  However, this leads to strange renderings,  
especially when scrolling mechanisms are present.  The partial  
overlapping also makes it more likely that scroll blitting will have  
to be disabled.

I'd prefer to see overflow special-cased such that it does not  
interleave with other normal flow content by default.  I don't think  
there is any real benefit to be gained by allowing such interleaving.  
It is also more difficult to encapsulate the painting of the overflow  
(e.g., if you wanted to optimize it for accelerated compositing) if  
the overflow block has to break up its painting because of surrounding  
normal flow content.

The above test case does support interleaving in IE8 (standards mode),  
Firefox and Opera.  Interleaving is not the behavior you get in IE7 or  
WebKit.

If this idea appeals to others, I can work on producing the new  
language for

http://www.w3.org/TR/CSS21/zindex.html

dave
(hyatt@apple.com)