Fix sticky constraints and update sticky layer positions recursively after scroll.

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 cb416dca18088a6b66608b98352cce926530b206..2139534693ef16d6708ac98260548db8864250c9 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
@@ -632,7 +632,6 @@ LayoutSize LayoutBoxModelObject::relativePositionOffset() const
 
 void LayoutBoxModelObject::updateStickyPositionConstraints() const
 {
-    // TODO(flackr): This method is reasonably complicated and should have some direct unit testing.
     const FloatSize constrainingSize = computeStickyConstrainingRect().size();
 
     PaintLayerScrollableArea* scrollableArea = layer()->ancestorOverflowLayer()->getScrollableArea();
@@ -646,20 +645,31 @@ void LayoutBoxModelObject::updateStickyPositionConstraints() const
     }
     LayoutBox* scrollAncestor = layer()->ancestorOverflowLayer()->isRootLayer() ? nullptr : toLayoutBox(layer()->ancestorOverflowLayer()->layoutObject());
 
-    LayoutRect containerContentRect = containingBlock->contentBoxRect();
-    LayoutUnit maxWidth = containingBlock->availableLogicalWidth();
-
+    LayoutRect containerContentRect = containingBlock->layoutOverflowRect();
+    LayoutUnit maxContainerWidth = containingBlock->containingBlockLogicalWidthForContent();
     // 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.
+    LayoutUnit maxWidth = containingBlock->availableLogicalWidth();
+
+    // This is removing the padding of the containing block's overflow rect to get the flow
+    // box rectangle and removing the margin of the sticky element to ensure that space between
+    // the sticky element and its containing flow box. It is an open issue whether the margin
+    // should collapse (See https://www.w3.org/TR/css-position-3/#sticky-pos).
     containerContentRect.contractEdges(
-        minimumValueForLength(style()->marginTop(), maxWidth),
-        minimumValueForLength(style()->marginRight(), maxWidth),
-        minimumValueForLength(style()->marginBottom(), maxWidth),
-        minimumValueForLength(style()->marginLeft(), maxWidth));
+        minimumValueForLength(containingBlock->style()->paddingTop(), maxContainerWidth) + minimumValueForLength(style()->marginTop(), maxWidth),
+        minimumValueForLength(containingBlock->style()->paddingRight(), maxContainerWidth) + minimumValueForLength(style()->marginRight(), maxWidth),
+        minimumValueForLength(containingBlock->style()->paddingBottom(), maxContainerWidth) + minimumValueForLength(style()->marginBottom(), maxWidth),
+        minimumValueForLength(containingBlock->style()->paddingLeft(), maxContainerWidth) + minimumValueForLength(style()->marginLeft(), maxWidth));
 
     // Map to the scroll ancestor.
-    constraints.setScrollContainerRelativeContainingBlockRect(containingBlock->localToAncestorQuad(FloatRect(containerContentRect), scrollAncestor).boundingBox());
+    FloatRect scrollContainerRelativeContainingBlockRect(containingBlock->localToAncestorQuad(FloatRect(containerContentRect), scrollAncestor).boundingBox());
+    FloatSize scrollOffset(scrollAncestor ? toFloatSize(scrollAncestor->getScrollableArea()->adjustedScrollOffset()) : FloatSize());
+
+    // Include scroll offset in container position if the container is not our scroll ancestor.
+    if (containingBlock != scrollAncestor)

[chrishtr, 2016/06/16 11:03:59]

If containingBlock == scrollAncestor, scrollContainerRelativeContainingBlockRect
will not include a scroll offset, right?
IOW, I don't think the conditional on line 670 should be necessary, you can just
put line 671.

[flackr, 2016/06/29 13:21:06]

Correct.
Not true, because we don't want the containing block's rect to be shifted by its
scroll offset - it's box should be rooted at the top of its scroll at (0, 0).

+        scrollContainerRelativeContainingBlockRect.move(scrollOffset);
+    constraints.setScrollContainerRelativeContainingBlockRect(scrollContainerRelativeContainingBlockRect);
 
     FloatRect stickyBoxRect = isLayoutInline()
         ? FloatRect(toLayoutInline(this)->linesBoundingBox())
@@ -671,13 +681,12 @@ void LayoutBoxModelObject::updateStickyPositionConstraints() const
     // TODO(flackr): Unfortunate to call localToAncestorQuad again, but we can't just offset from the previously computed rect if there are transforms.
     // Map to the scroll ancestor.
     FloatRect scrollContainerRelativeContainerFrame = containingBlock->localToAncestorQuad(FloatRect(FloatPoint(), FloatSize(containingBlock->size())), scrollAncestor).boundingBox();
+    scrollContainerRelativeContainerFrame.move(scrollOffset);
 
     // If the containing block is our scroll ancestor, its location will not include the scroll offset which we need to include as
     // part of the sticky box rect so we include it here.
-    if (containingBlock->hasOverflowClip()) {
-        FloatSize scrollOffset(toFloatSize(containingBlock->layer()->getScrollableArea()->adjustedScrollOffset()));
+    if (containingBlock == scrollAncestor)
         stickyLocation -= scrollOffset;
-    }
 
     constraints.setScrollContainerRelativeStickyBoxRect(FloatRect(scrollContainerRelativeContainerFrame.location() + toFloatSize(stickyLocation), flippedStickyBoxRect.size()));
 
@@ -735,7 +744,7 @@ FloatRect LayoutBoxModelObject::computeStickyConstrainingRect() const
 
     LayoutBox* enclosingClippingBox = toLayoutBox(layer()->ancestorOverflowLayer()->layoutObject());
     FloatRect constrainingRect;
-    constrainingRect = FloatRect(enclosingClippingBox->overflowClipRect(LayoutPoint()));
+    constrainingRect = FloatRect(enclosingClippingBox->overflowClipRect(LayoutPoint(DoublePoint(enclosingClippingBox->getScrollableArea()->adjustedScrollOffset()))));
     constrainingRect.move(enclosingClippingBox->paddingLeft(), enclosingClippingBox->paddingTop());
     constrainingRect.contract(FloatSize(enclosingClippingBox->paddingLeft() + enclosingClippingBox->paddingRight(),
         enclosingClippingBox->paddingTop() + enclosingClippingBox->paddingBottom()));