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());

[fs, 2017/04/29 09:59:06]

Do you even need this? Your bit should essentially be "Inherited: yes" (by
regular CSS spec parlance), so this bit would be updated by
ComputedStyle::InheritFrom regardless. (Make test if unsure.)

[smcgruer, 2017/05/01 20:15:15]

Done.

+}
+
 void StyleBuilderFunctions::applyInheritCSSPropertyPosition(
     StyleResolverState& state) {
   if (!state.ParentNode()->IsDocumentNode())
     state.Style()->SetPosition(state.ParentStyle()->GetPosition());
+  state.Style()->SetIsInStickySubtree(state.ParentStyle()->IsInStickySubtree());

[fs, 2017/04/29 09:59:06]

Ditto.

[smcgruer, 2017/05/01 20:15:15]

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 ||

[fs, 2017/04/29 12:10:31]

Thinking about it some more, maybe this should rather go in StyleAdjuster (since
that can override/change 'position', and you want to reduce the pessimism in
this flag much as possible I think.)

[smcgruer, 2017/05/01 20:15:15]

Done. Based on my tests it appears to work, so I assume StyleAdjuster is called
recursively if a parent style changes?

[fs, 2017/05/01 21:19:33]

Yes, since the flag is part of (rare) inherited, any difference to it should
cause a change (to 'position' essentially) to trigger recalc for the subtree.
CompustedStyle::StylePropagationDiff and NonIndependentInheritedEqual being the
key ingredients I believe. (Naturally this might also be a cause of alarm/new
perf differences...)

+                                      state.ParentStyle()->IsInStickySubtree());
 }
 
 void StyleBuilderFunctions::applyInitialCSSPropertyCaretColor(