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 cd3587ad63c36c9c01f3a8c43e808791cc8e3b93..88c5c2100c5faab146558b279f47232ccf229bac 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
@@ -635,7 +635,6 @@ 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();
@@ -649,20 +648,26 @@ 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.

[chrishtr, 2016/06/13 19:46:39]

Is this comment obsolete now that you are calling
containingBlockLogicalWidthForContent?

[flackr, 2016/06/15 13:45:49]

No, we're only calling containingBlockLogicalWidthForContent on the container
(for resolving sizes on the container), but not on the sticky element's
container (for resolving relative sizes on the sticky element).

     // Compute the container-relative area within which the sticky element is allowed to move.
+    LayoutUnit maxWidth = containingBlock->availableLogicalWidth();

[chrishtr, 2016/06/13 19:46:39]

Add a comment explaining that this is padding of containing block plus margin of
sticky element,
with link to spec.

[flackr, 2016/06/15 13:45:49]

Done.

     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)
+        scrollContainerRelativeContainingBlockRect.move(scrollOffset);
+    constraints.setScrollContainerRelativeContainingBlockRect(scrollContainerRelativeContainingBlockRect);
 
     FloatRect stickyBoxRect = isLayoutInline()
         ? FloatRect(toLayoutInline(this)->linesBoundingBox())
@@ -674,13 +679,12 @@ 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()));
 
@@ -738,7 +742,7 @@ 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()));