Re: Proposal for overflow painting order

On Feb 4, 2009, at 5:48 PM, Robert O'Callahan wrote:

> On Thu, Feb 5, 2009 at 11:55 AM, David Hyatt <hyatt@apple.com> wrote:
> 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.
>
> I'm not really sure what the last sentence means. In Gecko, we do  
> quite a bit of analysis to figure out if blitting is possible; for  
> example, we disallow blitting if the scrollable element is  
> transparent and the content behind it is not a single solid color,

Sure, but in an accelerated compositing world, you can imagine the  
overflow object having an offscreen buffer associated with it.  In the  
example you just cited, blitting would still be possible, since you  
would be able to blit in the buffer itself when scrolling happens.  It  
wouldn't matter that the scrollable element was transparent or that  
the content behind is not a single solid color.

However in the example of content from outside the overflow section  
actually intruding into and interleaving with the painting of the  
overflow section, you can't do this.

I also think authors using negative margins could cause unintentional  
overlap that could negatively impact performance of these overflow  
sections.

dave
(hyatt@apple.com)