Add offset contributed to sticky position box rect by location containers

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 486a11a43eb50a95648e8c40706d57d7d6d7f796..e36f404c456f2cbab52e4a9af977c6604214ec14 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
@@ -774,6 +774,15 @@ void LayoutBoxModelObject::updateStickyPositionConstraints() const {
   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 +=