Bug 31283 – position:fixed and transform on same element breaks fixed behavior

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

[Simon Fraser (smfr)]

Created attachment 42844 [details]
Testcase

If an element has both position:fixed and a transform, then it behaves as if it's absolutely positioned.

I think the fixed positioning should take effect, and then the transform.

[Simon Fraser (smfr)]

Created attachment 45920 [details]
Patch

Comment on attachment 45920 [details]
Patch

> @@ -982,9 +984,14 @@ void RenderBox::mapAbsoluteToLocalPoint(bool fixed, bool useTransforms, Transfor
>      if (style()->position() == FixedPosition)
>          fixed = true;
>  
> +    bool isFixedPos = style()->position() == FixedPosition;
>      bool hasTransform = hasLayer() && layer()->transform();
> -    if (hasTransform)
> -        fixed = false;  // Elements with transforms act as a containing block for fixed position descendants
> +    if (hasTransform) {
> +        // If this box has a transform, it acts as a fixed position container for fixed descendants,
> +        // and may itself also be fixed position. So propagate 'fixed' up only if this box is fixed position.
> +        fixed &= isFixedPos;
> +    } else
> +        fixed |= isFixedPos;

This doesn’t look right.

[Simon Fraser (smfr)]

Created attachment 45943 [details]
Revised patch

[Eric Seidel (no email)]

This patch is missing the binary data, which is why it's failing to apply correctly.  Note that "webkit-patch upload" or "webkit-patch post" or "webkit-patch post-commits" all know how to generate git diff --binary patches correctly so that the bots will be able to property process them. :)

[Simon Fraser (smfr)]

Created attachment 46404 [details]
Patch

Comment on attachment 46404 [details]
Patch

> +    EPosition position;
> +    if ((position = renderer()->style()->position()) == FixedPosition) {
> +        if (!ancestorLayer || ancestorLayer == renderer()->view()->layer()) {

Please just initialize position first, then do a single if statement:
if (position == FixedPosition && (!ancestorLayer || ancestorLayer == renderer()->view()->layer())) {
…

r=me

[Simon Fraser (smfr)]

http://trac.webkit.org/changeset/53173

[Simon Hausmann]

cherry-pick-for-backport: <r53173>

[Simon Hausmann]

Revision r53173 cherry-picked into qtwebkit-2.0 with commit 08dfb1202a63ed6d24be50484e3fbf8bbb9697a4