Clean compositing inputs for location APIs for sticky-affected elements.

third_party/WebKit/Source/core/css/resolver/StyleBuilderCustom.cpp

Index: third_party/WebKit/Source/core/css/resolver/StyleBuilderCustom.cpp
diff --git a/third_party/WebKit/Source/core/css/resolver/StyleBuilderCustom.cpp b/third_party/WebKit/Source/core/css/resolver/StyleBuilderCustom.cpp
index b44925bed08f099a2ad791ae7fb6780cdcfc35e9..38515931c17f1dc4006c96b4eb4bf5ebafed77fc 100644
--- a/third_party/WebKit/Source/core/css/resolver/StyleBuilderCustom.cpp
+++ b/third_party/WebKit/Source/core/css/resolver/StyleBuilderCustom.cpp
@@ -966,10 +966,26 @@ void StyleBuilderFunctions::applyValueCSSPropertyBaselineShift(
   }
 }
 
+void StyleBuilderFunctions::applyInitialCSSPropertyPosition(
+    StyleResolverState& state) {
+  state.Style()->SetPosition(ComputedStyle::InitialPosition());
+  state.Style()->SetIsInStickySubtree(state.ParentStyle()->IsInStickySubtree());
+}
+
 void StyleBuilderFunctions::applyInheritCSSPropertyPosition(
     StyleResolverState& state) {
   if (!state.ParentNode()->IsDocumentNode())
     state.Style()->SetPosition(state.ParentStyle()->GetPosition());
+  state.Style()->SetIsInStickySubtree(state.ParentStyle()->IsInStickySubtree());

[flackr, 2017/04/27 15:50:02]

I patched this in and I only see
StyleBuilderFunctions::applyValueCSSPropertyPosition being called, and only on
the elements which have had position explicitly set on them (i.e. #a and #b on
http://output.jsbin.com/gamisi/quiet). Are you sure it's inheriting correctly?

[smcgruer, 2017/04/27 16:05:28]

Did you set position to inherit? That is the case where this method would be
called.

[flackr, 2017/04/27 17:17:01]

But if the methods don't get called automatically on the descendants then won't
we have the same bug if you ask for the offset of a descendant of a sticky
element? (i.e. we won't have updated the IsInStickySubtree property to know
that).

[flackr, 2017/04/27 17:20:53]

Looks like it doesn't get inherited to the descendant - see
http://output.jsbin.com/gamisi/quiet

[flackr, 2017/04/27 19:20:51]

Nevermind I was mistakenly assuming the offsetTop of the child should change,
and you do have a test for the child in ElementTest.cpp. Can you add a couple
tests to make sure adding and removing isInStickySubtree is updated correctly
for the correct elements?

[smcgruer, 2017/04/28 18:20:37]

Done.

+}
+
+void StyleBuilderFunctions::applyValueCSSPropertyPosition(
+    StyleResolverState& state,
+    const CSSValue& value) {
+  EPosition position = ToCSSIdentifierValue(value).ConvertTo<EPosition>();
+  state.Style()->SetPosition(position);
+  state.Style()->SetIsInStickySubtree(position == EPosition::kSticky ||
+                                      state.ParentStyle()->IsInStickySubtree());
 }
 
 void StyleBuilderFunctions::applyInitialCSSPropertyCaretColor(