Adapt and reland old position sticky implementation

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp

Index: third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
index 06b747857bf5c636f032f9636b2e59c468b49fec..c19c9e44fb1ad291cdbe3d463f93625435effb5a 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
@@ -178,6 +178,21 @@ void LayoutBoxModelObject::styleWillChange(StyleDifference diff, const ComputedS
     LayoutObject::styleWillChange(diff, newStyle);
 }
 
+static inline LayoutBox* findScrollAncestor(const Node* startNode)
+{
+    ASSERT(startNode->layoutObject());
+    LayoutBox* curBox = startNode->layoutObject()->containingBlock();
+
+    // Scrolling propagates along the containing block chain.
+    while (curBox && !curBox->isLayoutView()) {
+        if (curBox->hasOverflowClip())
+            return curBox;
+        curBox = curBox->containingBlock();
+    }
+
+    return nullptr;
+}
+
 void LayoutBoxModelObject::styleDidChange(StyleDifference diff, const ComputedStyle* oldStyle)
 {
     bool hadTransform = hasTransformRelatedProperty();
@@ -268,8 +283,17 @@ void LayoutBoxModelObject::styleDidChange(StyleDifference diff, const ComputedSt
     }
 
     if (FrameView *frameView = view()->frameView()) {
-        bool newStyleIsViewportConstained = style()->hasViewportConstrainedPosition();
-        bool oldStyleIsViewportConstrained = oldStyle && oldStyle->hasViewportConstrainedPosition();
+        bool newStyleIsViewportConstained = style()->position() == FixedPosition;
+        bool oldStyleIsViewportConstrained = oldStyle && oldStyle->position() == FixedPosition;
+
+        // Sticky positioned elements are only viewport constrained if they have no ancestor scroller.
+        if (style()->position() == StickyPosition || (oldStyle && oldStyle->position() == StickyPosition)) {
+            if (!findScrollAncestor(node())) {
+                newStyleIsViewportConstained |= style()->position() == StickyPosition;
+                oldStyleIsViewportConstrained |= oldStyle && oldStyle->position() == StickyPosition;
+            }
+        }
+
         if (newStyleIsViewportConstained != oldStyleIsViewportConstrained) {
             if (newStyleIsViewportConstained && layer())
                 frameView->addViewportConstrainedObject(this);
@@ -608,6 +632,118 @@ LayoutSize LayoutBoxModelObject::relativePositionOffset() const
     return offset;
 }
 
+void LayoutBoxModelObject::computeStickyPositionConstraints(StickyPositionViewportConstraints& constraints, const LayoutRect& constrainingRect) const

[chrishtr, 2015/12/09 00:37:38]

I have a bunch of questions about this method. The best way forward though is to
do a VC to discuss in person, which will make the review much faster. Could you
schedule a meeting for tomorrow?

[flackr, 2015/12/10 23:43:15]

Acknowledged.

+{
+    LayoutBlock* containingBlock = this->containingBlock();
+
+    LayoutRect containerContentRect = containingBlock->contentBoxRect();
+    LayoutUnit maxWidth = containingBlock->availableLogicalWidth();
+
+    // Sticky positioned element ignore any override logical width on the containing block (as they don't call
+    // containingBlockLogicalWidthForContent). It's unclear whether this is totally fine.
+    // Compute the container-relative area within which the sticky element is allowed to move.
+    containerContentRect.contractEdges(
+        minimumValueForLength(style()->marginTop(), maxWidth),
+        minimumValueForLength(style()->marginRight(), maxWidth),
+        minimumValueForLength(style()->marginBottom(), maxWidth),
+        minimumValueForLength(style()->marginLeft(), maxWidth));
+
+    // Map to the view to avoid including page scale factor.
+    constraints.setAbsoluteContainingBlockRect(LayoutRect(containingBlock->localToContainerQuad(FloatRect(containerContentRect), view()).boundingBox()));
+
+    LayoutRect stickyBoxRect = isLayoutInline()
+        ? LayoutRect(toLayoutInline(this)->linesBoundingBox())
+        : toLayoutBox(this)->frameRect();
+    LayoutRect flippedStickyBoxRect = stickyBoxRect;
+    containingBlock->flipForWritingMode(flippedStickyBoxRect);
+    LayoutPoint stickyLocation = flippedStickyBoxRect.location();
+
+    // FIXME: sucks to call localToAbsolute again, but we can't just offset from the previously computed rect if there are transforms.
+    // Map to the view to avoid including page scale factor.
+    LayoutRect absContainerFrame = LayoutRect(containingBlock->localToContainerQuad(FloatRect(FloatPoint(), FloatSize(containingBlock->size())), view()).boundingBox());
+
+    if (containingBlock->hasOverflowClip()) {
+        LayoutSize scrollOffset(containingBlock->layer()->scrollableArea()->adjustedScrollOffset());

[chrishtr, 2015/12/09 19:12:21]

Is this code useless? Can the containing block be a scroller without also being
the sticky scroll container?

[flackr, 2015/12/10 23:43:15]

No, the containing block can't be a scroller without also being the sticky
scroll container afaik, but this is necessary. If the containing block is also
the scroller, absContainerFrame.location() is the location of the scroller which
does not include the scroll offset. When we go to set the absoluteStickyBoxRect
it then thinks that it's always at the same "absolute" position regardless of
how much the container has been scrolled.

[chrishtr, 2015/12/11 02:06:11]

Ok please add a comment explaining this then.

[flackr, 2016/01/19 15:40:48]

Done.

+        stickyLocation -= scrollOffset;
+    }
+
+    // We can't call localToAbsolute on |this| because that will recur. FIXME: For now, assume that |this| is not transformed.
+    constraints.setAbsoluteStickyBoxRect(LayoutRect(LayoutPoint(absContainerFrame.location()) + stickyLocation, flippedStickyBoxRect.size()));
+
+    LayoutUnit horizontalOffsets = minimumValueForLength(style()->right(), constrainingRect.width()) +
+        minimumValueForLength(style()->left(), constrainingRect.width());
+    bool skipRight = false;
+    bool skipLeft = false;
+    if (!style()->left().isAuto() && !style()->right().isAuto()) {
+        if (horizontalOffsets > containerContentRect.width()
+            || horizontalOffsets + containerContentRect.width() > constrainingRect.width()) {
+            skipRight = style()->isLeftToRightDirection();
+            skipLeft = !skipRight;
+        }
+    }
+
+    if (!style()->left().isAuto() && !skipLeft) {
+        constraints.setLeftOffset(minimumValueForLength(style()->left(), constrainingRect.width()));
+        constraints.addAnchorEdge(ViewportConstraints::AnchorEdgeLeft);
+    }
+
+    if (!style()->right().isAuto() && !skipRight) {
+        constraints.setRightOffset(minimumValueForLength(style()->right(), constrainingRect.width()));
+        constraints.addAnchorEdge(ViewportConstraints::AnchorEdgeRight);
+    }
+
+    bool skipBottom = false;
+    // FIXME(ostap): Exclude top or bottom edge offset depending on the writing mode when related
+    // sections are fixed in spec: http://lists.w3.org/Archives/Public/www-style/2014May/0286.html
+    LayoutUnit verticalOffsets = minimumValueForLength(style()->top(), constrainingRect.width()) +
+        minimumValueForLength(style()->bottom(), constrainingRect.width());
+    if (!style()->top().isAuto() && !style()->bottom().isAuto()) {
+        if (verticalOffsets > containerContentRect.height()
+            || verticalOffsets + containerContentRect.height() > constrainingRect.height()) {
+            skipBottom = true;
+        }
+    }
+
+    if (!style()->top().isAuto()) {
+        constraints.setTopOffset(minimumValueForLength(style()->top(), constrainingRect.height()));
+        constraints.addAnchorEdge(ViewportConstraints::AnchorEdgeTop);
+    }
+
+    if (!style()->bottom().isAuto() && !skipBottom) {
+        constraints.setBottomOffset(minimumValueForLength(style()->bottom(), constrainingRect.height()));
+        constraints.addAnchorEdge(ViewportConstraints::AnchorEdgeBottom);
+    }
+}
+
+LayoutRect LayoutBoxModelObject::computeStickyConstrainingRect() const
+{
+    LayoutRect constrainingRect;
+
+    ASSERT(hasLayer());
+    LayoutBox* enclosingClippingBox = findScrollAncestor(node());
+    if (enclosingClippingBox) {
+        LayoutRect clipRect = enclosingClippingBox->overflowClipRect(LayoutPoint());
+        clipRect.move(enclosingClippingBox->paddingLeft(), enclosingClippingBox->paddingTop());
+        clipRect.contract(LayoutSize(enclosingClippingBox->paddingLeft() + enclosingClippingBox->paddingRight(),

[chrishtr, 2015/12/09 19:12:21]

Why is special code needed to remove padding?

[flackr, 2015/12/10 23:43:15]

The padding is not clipped from an overflow box, so we need to remove it here.
In the spec it says that we stick to the flow box rect which I think means the
rect in which the scroller's content flows in, and this is consistent with how
its implemented by Firefox and Safari.

+            enclosingClippingBox->paddingTop() + enclosingClippingBox->paddingBottom()));
+        constrainingRect = LayoutRect(enclosingClippingBox->localToContainerQuad(FloatRect(clipRect), view()).boundingBox());
+    } else {
+        constrainingRect = LayoutRect(view()->frameView()->visibleContentRect());
+    }
+
+    return constrainingRect;
+}
+
+LayoutSize LayoutBoxModelObject::stickyPositionOffset() const
+{
+    LayoutRect constrainingRect = computeStickyConstrainingRect();
+    StickyPositionViewportConstraints constraints;
+    computeStickyPositionConstraints(constraints, constrainingRect);
+
+    // The sticky offset is physical, so we can just return the delta computed in absolute coords (though it may be wrong with transforms).
+    return LayoutSize(constraints.computeStickyOffset(constrainingRect));
+}
+
 LayoutPoint LayoutBoxModelObject::adjustedPositionRelativeToOffsetParent(const LayoutPoint& startPoint) const
 {
     // If the element is the HTML body element or doesn't have a parent
@@ -630,7 +766,7 @@ LayoutPoint LayoutBoxModelObject::adjustedPositionRelativeToOffsetParent(const L
             referencePoint.move(-toLayoutBox(offsetParent)->borderLeft(), -toLayoutBox(offsetParent)->borderTop());
         if (!isOutOfFlowPositioned() || flowThreadContainingBlock()) {
             if (isInFlowPositioned())
-                referencePoint.move(relativePositionOffset());
+                referencePoint.move(offsetForInFlowPosition());
 
             LayoutObject* current;
             for (current = parent(); current != offsetParent && current->parent(); current = current->parent()) {
@@ -652,7 +788,13 @@ LayoutPoint LayoutBoxModelObject::adjustedPositionRelativeToOffsetParent(const L
 
 LayoutSize LayoutBoxModelObject::offsetForInFlowPosition() const
 {
-    return isRelPositioned() ? relativePositionOffset() : LayoutSize();
+    if (isRelPositioned())
+        return relativePositionOffset();
+
+    if (isStickyPositioned())
+        return stickyPositionOffset();
+
+    return LayoutSize();
 }
 
 LayoutUnit LayoutBoxModelObject::offsetLeft() const
@@ -975,7 +1117,7 @@ const LayoutObject* LayoutBoxModelObject::pushMappingToContainer(const LayoutBox
         return nullptr;
 
     bool isInline = isLayoutInline();
-    bool isFixedPos = !isInline && style()->position() == FixedPosition;
+    bool isFixedPosition = !isInline && style()->position() == FixedPosition;
     bool hasTransform = !isInline && hasLayer() && layer()->transform();
 
     LayoutSize adjustmentForSkippedAncestor;
@@ -987,16 +1129,19 @@ const LayoutObject* LayoutBoxModelObject::pushMappingToContainer(const LayoutBox
 
     bool offsetDependsOnPoint = false;
     LayoutSize containerOffset = offsetFromContainer(container, LayoutPoint(), &offsetDependsOnPoint);
+    LayoutSize stickyOffset;
+    if (style()->position() == StickyPosition)
+        stickyOffset = offsetForInFlowPosition();
 
     bool preserve3D = container->style()->preserves3D() || style()->preserves3D();
     if (shouldUseTransformFromContainer(container)) {
         TransformationMatrix t;
         getTransformFromContainer(container, containerOffset, t);
         t.translateRight(adjustmentForSkippedAncestor.width().toFloat(), adjustmentForSkippedAncestor.height().toFloat());
-        geometryMap.push(this, t, preserve3D, offsetDependsOnPoint, isFixedPos, hasTransform);
+        geometryMap.push(this, t, preserve3D, offsetDependsOnPoint, isFixedPosition, hasTransform, LayoutSize(), stickyOffset);
     } else {
         containerOffset += adjustmentForSkippedAncestor;
-        geometryMap.push(this, containerOffset, preserve3D, offsetDependsOnPoint, isFixedPos, hasTransform);
+        geometryMap.push(this, containerOffset, preserve3D, offsetDependsOnPoint, isFixedPosition, hasTransform, LayoutSize(), stickyOffset);
     }
 
     return ancestorSkipped ? ancestorToStopAt : container;