[webkit-dev] Fwd: hasStaticBlockPosition in RenderBlock::layoutPositionedObjects(bool relayoutChildren)

[Sravan]

Hi,

Can some one please tell me who can clarify my doubts on Layout of
Positioned Objects, please?

-Sravan.

---------- Forwarded message ----------
From: Sravan <sra1sandela at gmail.com>
Date: Fri, May 18, 2012 at 4:20 PM
Subject: hasStaticBlockPosition in
RenderBlock::layoutPositionedObjects(bool relayoutChildren)
To: webkit-help at lists.webkit.org


Hello WebKittens,

In

RenderBlock::layoutPositionedObjects(bool relayoutChildren)

following code is actually supposed to setChildNeedsLayout to only those
positioned objects. When a non-positioned block element moves, and might
have positioned children that are implicitly positioned relative to
the non-positioned block.

        // When a non-positioned block element moves, it may have
positioned children that are implicitly positioned relative to the
        // non-positioned block.  Rather than trying to detect all of these
movement cases, we just always lay out positioned
        // objects that are positioned implicitly like this.  Such objects
are rare, and so in typical DHTML menu usage (where everything is
        // positioned explicitly) this should not incur a performance
penalty.
        if (relayoutChildren ||
(r->style()->hasStaticBlockPosition(isHorizontalWritingMode()) &&
r->parent() != this))
            r->setChildNeedsLayout(true, MarkOnlyThis);

But, it looks like it is accidentally calling setChildNeedsLayout even on
those objects which has Absolute position mentioned exclusively through css.
And as

hasStaticBlockPosition(isHorizontalWritingMode())

checks for hasAutoTopAndBottom() in horizontalMode, when we provide
"top/bottom" attributes to any Absolute Positioned element through css,
then setChildNeedsLayout wont be called and hence we are seeing incorrect
layout.

I am able to fix this by editing hasAutoTopAndBottom() as follows

bool hasAutoTopAndBottom() const { return (top().isAuto() ||
top().isFixed()) && (bottom().isAuto() || bottom().isFixed()); }

but the implementation does'nt really comply with the name, so either my
fix has to get a different function with a fix something similar as above,
or have a separate if block in RenderBlock::layoutPositionedObjects with an
entirely different implementation .

So, i do not want to submit a patch which will surely get a r-, and waste
reviewers time, also i would like to clarify on the bigger issue of
accidentally laying out positioned elements which the comment does'nt
really reflect.

*P.S: If by any chance i have successfully managed to confuse the reader,
please go through https://bugs.webkit.org/show_bug.cgi?id=80808 for the
use-case and relevant discussion.*
*
*
*I am not able to find a relevant person to discuss about this on IRC. so,
please CC yourself on that bug, and please comment there to continue the
thread.*

-Sravan.
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