Bug 217154 – Absolutely positioned and negative z-index div with canvas child gets drawn with wrong stacking order

• Component: Compositing
• Status: RESOLVED FIXED

[Yong Su]

Created attachment 410171 [details]
Screenshot of incorrect stacking order

Steps to reproduce:
1. Navigate to http://jsfiddle.net/jeantimex/Lsvyz0nj/
2. Notice that there are two divs (blue and red)

What to expect:
The red div should stack above the blue div.

Actual result:
The blue div stacks above the red div.

Comments:
Both blue and red divs are positioned absolutely, the blue div has a canvas child. Both divs have the same negative z-index.

This bug does not happen on Chrome, Firefox, IE, Edge. It affects all Safari browsers on all platforms.

Workarounds:
1. Change both z-index to positive, e.g., 1 (however, in some business cases, it's expected both divs have the same negative z-index).
2. Change the red view z-index to 0. (however, in some business cases, it's expected both divs have the same negative z-index).
3. Set transform: translateZ(0) or translate3d(0, 0, 0) to the red div. (I guess this will force a paint for the red div in a different layer)

[Radar WebKit Bug Importer]

<rdar://problem/69818585>

[Rob Buis]

Created attachment 436847 [details]
Patch

[Rob Buis]

Created attachment 436891 [details]
Patch

[Simon Fraser (smfr)]

Comment on attachment 436891 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=436891&action=review

> LayoutTests/ChangeLog:11
> +        * compositing/composited-canvas-overlaps-absolute-div-expected.html: Added.
> +        * compositing/composited-canvas-overlaps-absolute-div.html: Added.

I think the test name should have "multiple-negative-z" in it since that's key to the bug. And the fact that it's canvas isn't relevant (in fact, it would be better to make a test that doesn't use canvas).

[Darin Adler]

Comment on attachment 436891 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=436891&action=review

> Source/WebCore/rendering/RenderLayerCompositor.cpp:1101
> +    Vector<RenderLayer*> layerWillCompositeNeeded;

If this typically is a small set of layers, we should optimize by adding inline capacity:

    Vector<RenderLayer*, 2> ...

We just need a small number that is almost always higher than the number of layers.

> Source/WebCore/rendering/RenderLayerCompositor.cpp:1111
> +        if (childLayer) {

This isn’t needed. We would be adding null layer pointers to the vector.

[Rob Buis]

Created attachment 437002 [details]
Patch

[Rob Buis]

Comment on attachment 436891 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=436891&action=review

>> Source/WebCore/rendering/RenderLayerCompositor.cpp:1101
>> +    Vector<RenderLayer*> layerWillCompositeNeeded;
> 
> If this typically is a small set of layers, we should optimize by adding inline capacity:
> 
>     Vector<RenderLayer*, 2> ...
> 
> We just need a small number that is almost always higher than the number of layers.

Fair enough, however I now use a simple counter, since we do not need to actually remember the RenderLayers, just how many.

>> Source/WebCore/rendering/RenderLayerCompositor.cpp:1111
>> +        if (childLayer) {
> 
> This isn’t needed. We would be adding null layer pointers to the vector.

See above.

>> LayoutTests/ChangeLog:11
>> +        * compositing/composited-canvas-overlaps-absolute-div.html: Added.
> 
> I think the test name should have "multiple-negative-z" in it since that's key to the bug. And the fact that it's canvas isn't relevant (in fact, it would be better to make a test that doesn't use canvas).

Done.

[Darin Adler]

Comment on attachment 437002 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=437002&action=review

> Source/WebCore/rendering/RenderLayerCompositor.cpp:1101
> +    unsigned layerWillCompositeNeeded = 0;

Style thing: Typically it’s better to name a count with a noun like "number" or "count". A phrase like this can sound like it might be a set, not an integer. Maybe "numberOfLayerWillCompositeCallsNeeded", but that’s a bit long. Maybe "newlyCompositedChildLayerCount"?

Out of my depth, I guess, but I’ve got to admit, I am puzzled about why we need to call layerWillComposite over and over again.

[Rob Buis]

Created attachment 437119 [details]
Patch

[EWS]

Committed r281913 (?): <https://commits.webkit.org/r281913>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 437119 [details].