Bug 16328 - Use of "containing block" does not match CSS2.1 definition
Use of "containing block" does not match CSS2.1 definition
Status: NEW
Product: CSS
Classification: Unclassified
Component: Transforms
All All
: P2 normal
: ---
Assigned To: Simon Fraser
: 18500 19637 (view as bug list)
Depends on:
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Reported: 2012-03-12 19:55 UTC by Aryeh Gregor
Modified: 2014-08-11 17:36 UTC (History)
8 users (show)

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Description Aryeh Gregor 2012-03-12 19:55:17 UTC
See Mozilla bug: https://bugzilla.mozilla.org/show_bug.cgi?id=733849

The spec says:

Elements establish and participate in 3D rendering contexts as follows:

* A 3D rendering context is established by a a transformable element whose computed value for ‘transform-style’ is ‘preserve-3d’, and which itself is not part of a 3D rendering context. Note that such an element is always a containing block. An element that establishes a 3D rendering context also participates in that context.
* An element whose computed value for ‘transform-style’ is ‘preserve-3d’, and which itself participates in a 3D rendering context, extends that 3D rendering context rather than establishing a new one.
* An element participates in a 3D rendering context if its containing block establishes or extends a 3D rendering context.

One problem here is that a containing block is not always an element.  For instance, in the markup

  <div><img src=foo><div>More text</div></div>

the img's containing block is an anonymous block box.  What does it mean for that box to establish or extend a 3D rendering context?  If the outer div has transform-style: preserve-3d, is the img part of a 3D rendering context or not?  More concretely, consider the following:

  data:text/html,<!doctype html>
  <div style="transform-style:preserve-3d;transform:rotatex(90deg)">
    <img src=image style="transform:rotatex(90deg)">
    <div>Some text</div>

In Chrome 19 dev (Linux), WebKit nightly r109732 (Windows 8), and Firefox 13.0a1, the image appears, so it is part of the rendering context.  How are we supposed to spec this?

Another problem is the statement "Note that such an element is always a containing block."  As Boris points out in <https://bugzilla.mozilla.org/show_bug.cgi?id=733849#c4>, this isn't always true.  According to <http://www.w3.org/TR/CSS2/visuren.html#block-boxes>, a table box isn't a block container box, for instance.  Also, per bug 16326, we want things like table rows to be transformable, but they aren't block container boxes.  Thus they can't be containing blocks.

I suggested the following wording, which doesn't use the concept of containing blocks at all:

Elements establish and participate in 3D rendering contexts as follows:

* If a transformable element's computed value for 'transform-style' is 'preserve-3d', and either its parent is not transformable or its parent's 'transform-style' computes to 'flat', it establishes a new 3D rendering context and participates in that context.

* If a transformable element's parent is transformable, and its parent's 'transform-style' computes to 'preserve-3d', it participates in the same 3D rendering context as its parent.

* If an element is not transformable, or its 'transform-style' and its parent's both compute to 'flat', or its 'transform-style' computes to 'flat' and its parent is not transformable, it does not participate in any 3D rendering context.

However, this doesn't match how browsers behave.  See the test-case in <https://bugzilla.mozilla.org/show_bug.cgi?id=733849#c7>.

I'm not sure what to spec here.  Any ideas?  What do Gecko and WebKit actually do?
Comment 1 Simon Fraser 2012-03-12 20:05:24 UTC
WebKit's behavior here is buggy and a side-effect of the implementation.

When I wrote that part of the spec I wasn't sure if 'containing block' was the right term to use. I'm not sure that 'parent' is either, since for an absolutely positioned element, what matters its its containing block, not its parent.
Comment 2 Aryeh Gregor 2012-03-12 20:23:42 UTC
Yes, my proposed definition changes behavior for absolute positioning.  Is this a problem, or is it a marginal enough corner case that we can just ask browsers to change?  If that's the only problem with my proposed definition, it could always be changed to have an extra special case for absolute positioning.
Comment 3 Boris Zbarsky 2012-03-12 22:06:11 UTC
The "containing block" is a rectangle; it's not an element or a box.  So talking about its styles is just nonsensical.

We really do need to decide what behavior we want in the anonymous block case and in the abspos/fixedpos cases before we can figure out how to define this....
Comment 4 Aryeh Gregor 2012-03-15 18:41:13 UTC
I think what we want here is to define some new term and use that instead of "containing block".  Like call it "transform parent", and say that the transform parent of an element is the nearest transformable ancestor if the element's position is relative or static, and the nearest ancestor with non-static position if the element's position is absolute, and the nearest ancestor that either is the root element or has a transform applied if the element's position is fixed.  All uses of "containing block" in the spec should be reviewed to see if they deserve to be changed.
Comment 5 Simon Fraser 2012-03-15 18:45:42 UTC
That sounds reasonable. I'd have to think about how to define "transform parent".
Comment 6 Boris Zbarsky 2012-03-15 18:52:41 UTC
The exact definition in comment 4 is not quite right (e.g. for position:absolute or position:fixed ancestors with transform not "none" should probably be treated as transform parents, and similar for svg:foreignObject), but the general approach makes sense, I think.  I wonder how to best keep the definition from getting out of sync with the containing block stuff for positioned elements...
Comment 7 Simon Fraser 2012-10-18 22:24:34 UTC
Need to discuss at the F2F meeting.
Comment 8 Edward O'Connor 2012-10-18 22:48:38 UTC
*** Bug 18500 has been marked as a duplicate of this bug. ***
Comment 9 Simon Fraser 2012-10-19 17:14:51 UTC
*** Bug 19637 has been marked as a duplicate of this bug. ***
Comment 10 Binyamin 2014-08-11 17:36:47 UTC
Chrome related bug "rendering bug : position:fixed AND -webkit-transform" https://code.google.com/p/chromium/issues/detail?id=20574