Bug 103329 – [CSS Grid Layout] Properly support for z-index on grid items

• Status: RESOLVED
• Resolution: FIXED
• Component: Layout and Rendering

[Julien Chaffraix]

Per specification, section 8 "Drawing Order of Grid Items", the z-index controls the drawing order. Also the grid element and grid items should generate stacking contexts

We should get most of the feature for free with having RenderLayers.

[Manuel Rego Casasnovas]

We would need to port the changes in Blink linked from this bug: https://code.google.com/p/chromium/issues/detail?id=234200

[Manuel Rego Casasnovas]

Created attachment 232211 [details]
Patch

[Build Bot]

Comment on attachment 232211 [details]
Patch

Attachment 232211 [details] did not pass mac-ews (mac):
Output: http://webkit-queues.appspot.com/results/4515958177660928

New failing tests:
css3/blending/repaint/blend-mode-turn-off-isolation.html

[Build Bot]

Created attachment 232217 [details]
Archive of layout-test-results from webkit-ews-02 for mac-mountainlion

The attached test failures were seen while running run-webkit-tests on the mac-ews.
Bot: webkit-ews-02  Port: mac-mountainlion  Platform: Mac OS X 10.8.5

[Build Bot]

Comment on attachment 232211 [details]
Patch

Attachment 232211 [details] did not pass mac-ews (mac):
Output: http://webkit-queues.appspot.com/results/5750338220982272

New failing tests:
css3/blending/repaint/blend-mode-turn-off-isolation.html

[Build Bot]

Created attachment 232219 [details]
Archive of layout-test-results from webkit-ews-08 for mac-mountainlion

The attached test failures were seen while running run-webkit-tests on the mac-ews.
Bot: webkit-ews-08  Port: mac-mountainlion  Platform: Mac OS X 10.8.5

[Build Bot]

Comment on attachment 232211 [details]
Patch

Attachment 232211 [details] did not pass mac-wk2-ews (mac-wk2):
Output: http://webkit-queues.appspot.com/results/6689821514792960

New failing tests:
css3/blending/repaint/blend-mode-turn-off-isolation.html

[Build Bot]

Created attachment 232233 [details]
Archive of layout-test-results from webkit-ews-14 for mac-mountainlion-wk2

The attached test failures were seen while running run-webkit-tests on the mac-wk2-ews.
Bot: webkit-ews-14  Port: mac-mountainlion-wk2  Platform: Mac OS X 10.8.5

[Manuel Rego Casasnovas]

Created attachment 233000 [details]
Patch

[Manuel Rego Casasnovas]

rosca, could you please take a look to the rebaseline for css3/blending/repaint/blend-mode-turn-off-isolation.html is it right? Thanks.

[Ion Rosca]

This shouldn't break anything on blending side. However, this is some extra paint effort on static elements forcing zIndex to 0 due to a 'isolation: isolate;' property.

[Manuel Rego Casasnovas]

Ping reviewers. Thanks.

[Benjamin Poulain]

Comment on attachment 233000 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=233000&action=review

> Source/WebCore/rendering/style/RenderStyle.cpp:676
> +    // StyleResolver has ensured that zIndex is non-auto only if it's applicable.

I would consider a series of assertions here to ensure that comment is true.
Otherwise a mistake in the future could cause us to overpaint like crazy.

> Source/WebCore/rendering/style/RenderStyle.cpp:678
> +    if (m_box->zIndex() != other->m_box->zIndex()
> +        || m_box->hasAutoZIndex() != other->m_box->hasAutoZIndex())

This can be on one line, no need to use Google's short line style.

> LayoutTests/ChangeLog:13
> +        * fast/css-grid-layout/grid-item-z-index-change-repaint-expected.html: Added.
> +        * fast/css-grid-layout/grid-item-z-index-change-repaint.html: Added.

I would add more tests similar to this:

Here you test if changing the z-index cause a correct repaint.

There is one area of the patch that is not tested by this: going in and out of grid position. I would add tests ensuring
1) An element supports the z-index once it gets a grid positioning.
2) It repaints accordingly.

[Manuel Rego Casasnovas]

Comment on attachment 233000 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=233000&action=review

Thanks for the review. I'm uploading a new version of the patch.

>> Source/WebCore/rendering/style/RenderStyle.cpp:676
>> +    // StyleResolver has ensured that zIndex is non-auto only if it's applicable.
> 
> I would consider a series of assertions here to ensure that comment is true.
> Otherwise a mistake in the future could cause us to overpaint like crazy.

I guess you're talking here about some assert checking that if z-index is not auto we're in positioned element or a grid item.
However, I haven't found how to know if we're in a grid item (in RenderStyle) as we would need to check the display value of its parent.
That's the reason why I'm not adding any assert in the new version of the patch.

[Manuel Rego Casasnovas]

Created attachment 233755 [details]
Patch

[Benjamin Poulain]

Comment on attachment 233755 [details]
Patch

Thanks!

[WebKit Commit Bot]

Comment on attachment 233755 [details]
Patch

Clearing flags on attachment: 233755

Committed r170474: <http://trac.webkit.org/changeset/170474>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.