This is a merge of http://trac.webkit.org/changeset/151623 by Simon Fraser <simon.fraser@apple.com>.

It was a mostly-mechanical merge. The reftest fixed-backgrounds.html required some tweaking,
since it relied on composited and non-composited output being identical. This required changing
the shadow from a soft shadow to a hard shadow.

BUG=330852

--

Source/WebCore: Painting of fixed background images is wrong in composited layers
​https://bugs.webkit.org/show_bug.cgi?id=65793
Reviewed by Sam Weinig.
The code that computed background image geometry for background-attachment:fixed
images was unaware of compositing, so often painting the image at the wrong location.
Fix by having RenderBoxModelObject::calculateBackgroundImageGeometry() do the correct
math for fixed backgrounds in composited layer by offsetting the viewport rect by
the paint container's absolute position.
Tests: compositing/backgrounds/fixed-background-on-descendant.html
compositing/backgrounds/fixed-backgrounds.html
rendering/RenderBox.cpp:
(WebCore::RenderBox::getBackgroundPaintedExtent): Now returns a bool indicating
whether it is returning a reliable extent rect. It can return false in the case where
a background is fixed, since computing the correct extent would require finding
the appropriate composited ancestor to pass to calculateBackgroundImageGeometry().
This is OK since this function is used for "background opaque" optimizations.
(WebCore::RenderBox::computeBackgroundIsKnownToBeObscured): If getBackgroundPaintedExtent()
returns false, return false.
(WebCore::RenderBox::maskClipRect): We removed mask-attachment, so we never need to
compute the composited ancestor here and can pass null.
(WebCore::RenderBox::repaintLayerRectsForImage): Unwrap a comment.
If the changed image is related to a fixed background, geometry.hasNonLocalGeometry()
will be true. In that cause, just repaint the entire renderer rather than groveling
around for a composited ancestor.
rendering/RenderBox.h: Changed name and signature of backgroundPaintedExtent.
rendering/RenderBoxModelObject.cpp:
(WebCore::RenderBoxModelObject::paintFillLayerExtended): calculateBackgroundImageGeometry()
now needs to know the painting container.
(WebCore::RenderBoxModelObject::calculateBackgroundImageGeometry): Now takes a painting
container, that is required to correctly compute the viewport-relative offset for fixed
backgrounds. geometry.setHasNonLocalGeometry() is set for fixed backgrounds to indicate
to callers that, if they didn't pass a paint container, the destRect is not accurate.
The main bug fix is also here: we move the viewportRect by the absolute location of
paint container, which is equivalent to the composited layer offset.
(WebCore::RenderBoxModelObject::getGeometryForBackgroundImage): calculateBackgroundImageGeometry()
takes a paint container.
rendering/RenderBoxModelObject.h:
(WebCore::RenderBoxModelObject::BackgroundImageGeometry::BackgroundImageGeometry):
(WebCore::RenderBoxModelObject::BackgroundImageGeometry::setHasNonLocalGeometry):
(WebCore::RenderBoxModelObject::BackgroundImageGeometry::hasNonLocalGeometry):
rendering/RenderImage.cpp:
(WebCore::RenderImage::computeBackgroundIsKnownToBeObscured): If getBackgroundPaintedExtent()
can't cheaply give an accurate answer, return false.
rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::updateDirectlyCompositedBackgroundImage): Pass the paint container,
which is our own renderer.
LayoutTests: Fixed background images behave strangely with webkit transitions
​https://bugs.webkit.org/show_bug.cgi?id=65793
Reviewed by Sam Weinig.
Ref tests that compare fixed background rendering after a scroll, with and
without compositing, with a couple of layer configurations.
compositing/backgrounds/fixed-background-on-descendant-expected.html: Added.
compositing/backgrounds/fixed-background-on-descendant.html: Added.
compositing/backgrounds/fixed-backgrounds-expected.html: Added.
compositing/backgrounds/fixed-backgrounds.html: Added.

Review URL: https://codereview.chromium.org/102123013

git-svn-id: svn://svn.chromium.org/blink/trunk@170618 bbb929c8-8fbe-4397-9dbb-9b2b20218538