Add offset contributed to sticky position box rect by location containers

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 755 matching lines @@
   return offset;
 }
 
 void LayoutBoxModelObject::updateStickyPositionConstraints() const {
   const FloatSize constrainingSize = computeStickyConstrainingRect().size();
 
   PaintLayerScrollableArea* scrollableArea =
       layer()->ancestorOverflowLayer()->getScrollableArea();
   StickyPositionScrollingConstraints constraints;
   FloatSize skippedContainersOffset;
   LayoutBlock* containingBlock = this->containingBlock();

[chrishtr, 2017/01/20 19:20:57]

Sort of unrelated to your CL:

LayoutObject::containingBlock says it returns a non-anonymous block.
Doesn't this make line 787 irrelevant?

[flackr, 2017/01/23 23:33:43]

Hmm, this was definitely necessary at the time (and has a test which should fail
if it's still necessary), I will try removing it.

+  // Add offset from containing block contributed by location containers.
+  if (!isLayoutInline()) {
+    LayoutBox* locationContainer = toLayoutBox(this)->locationContainer();
+    while (locationContainer && locationContainer != containingBlock) {

[chrishtr, 2017/01/20 19:20:57]

I think this and other code in this method may be duplicating logic already
elsewhere in
geometry-computation code. Adding Xianzhu to see if he has insights.

[Xianzhu, 2017/01/20 23:04:01]

I think localToAncestorPoint should work.

[flackr, 2017/01/23 23:33:43]

Sure, although I need a version that excludes transforms like I've done with the
containing block on 818-825.

[Xianzhu, 2017/01/24 00:00:39]

I'm not familiar with the code here. Can you explain why we need to exclude
transforms?

[flackr, 2017/01/24 22:37:11]

Sticky position offsets are computed in terms of layout position. If transform
affected it you end up with a lot of interesting and hard to reason about
problems (i.e. how do you stick with scale, rotation, perspective, etc), and it
would need to update in sync with composited animations. This is consistent with
how Firefox and mostly how Safari implements sticky position.

+      skippedContainersOffset +=
+          toFloatSize(FloatPoint(locationContainer->frameRect().location()));
+      locationContainer = locationContainer->locationContainer();
+    }
+  }
   // Skip anonymous containing blocks.
   while (containingBlock->isAnonymous()) {
     skippedContainersOffset +=
         toFloatSize(FloatPoint(containingBlock->frameRect().location()));
     containingBlock = containingBlock->containingBlock();
   }
   LayoutBox* scrollAncestor =
       layer()->ancestorOverflowLayer()->isRootLayer()
           ? nullptr
           : toLayoutBox(layer()->ancestorOverflowLayer()->layoutObject());
@@ skipping 553 matching lines @@
   if (rootElementStyle->hasBackground())
     return false;
 
   if (node() != document().firstBodyElement())
     return false;
 
   return true;
 }
 
 }  // namespace blink