[webkit-dev] RenderLayer::isStackingContext() confusion

[Shawn Singh]

Thanks for the explanation.  I submitted a patch, just in case reviewers
agree that a comment is appropriate there.  But I won't argue strongly
either way.  https://bugs.webkit.org/show_bug.cgi?id=113909


On Wed, Apr 3, 2013 at 1:18 PM, Elliott Sprehn <esprehn at chromium.org> wrote:

> The function does what it says, it's just not obvious why:
>
>
> https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/WebCore/css/StyleResolver.cpp&sq=package:chromium&type=cs&q=adjustRenderStyle&l=1469
>
> First we set the z-index to be auto for static position elements. Next we
> set the z-index to be 0 for elements based on the big set of cases in
> adjustRenderStyle, and then you can tell in RenderLayer if you're a
> stacking context based on if you have z-index != auto.
>
> I'd add a comment explaining. The alternative is to check that big set of
> cases every time someone calls isStackingContext but that might be slower.
> Changing the name of the method would make the code more confusing, since
> it is checking the right thing just through some complicated process.
>
>
> On Wed, Apr 3, 2013 at 3:48 PM, Shawn Singh <shawnsingh at chromium.org>wrote:
>
>> Background:
>>
>> According to CSS 2.1 spec, an element becomes a stacking context when:
>>   - it is positioned  (relative, absolute, or fixed)
>>   - and it has a non-auto z-index
>>
>> Spec also mentions that there's room for expanding this definition; the
>> examples I know about are fixed-position elements and elements with
>> transforms.
>>
>>
>> Questions:
>>
>> The implementation of RenderLayer::isStackingContext() looks fishy.  It
>> only checks for a non-auto z-index.  It does not check for the positioning
>> style of the layer.  There is no indication that it checks for any more
>> recent special cases such as fixed-position or transforms.
>>
>> Is this function name a misnomer?  should it actually be renamed to
>> hasNonAutoZIndex() ?
>>
>> If the additional criteria is not being checked inside this helper
>> function, then where is it?  I see some code in collectLayers that looks
>> vaguely similar to checking whether an element is positioned, but what
>> about all the other uses of isStackingContext everywhere?
>>
>> Thanks!
>> Shawn
>>
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>>
>>
>
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