Don't add siblingRules with combinator left of ::content.

third_party/WebKit/Source/core/css/RuleFeature.cpp

Index: third_party/WebKit/Source/core/css/RuleFeature.cpp
diff --git a/third_party/WebKit/Source/core/css/RuleFeature.cpp b/third_party/WebKit/Source/core/css/RuleFeature.cpp
index d732d585ab686d0a1b43c810e621cc032beb9b41..512207af67f2f0ddfc482ef28ad30283ce31a16c 100644
--- a/third_party/WebKit/Source/core/css/RuleFeature.cpp
+++ b/third_party/WebKit/Source/core/css/RuleFeature.cpp
@@ -587,15 +587,15 @@ void RuleFeatureSet::collectFeaturesFromSelector(const CSSSelector& selector, Ru
                 metadata.maxDirectAdjacentSelectors = maxDirectAdjacentSelectors;
             maxDirectAdjacentSelectors = 0;
         }
-        if (current->isSiblingSelector())
+        if (!metadata.foundInsertionPointCrossing && current->isSiblingSelector())

[kochi, 2016/02/15 04:10:17]

If foundInsertionPointCrossing is only used for the flag that indicate
::content is found in the loop, could it be a local boolean flag instead
of a FeatureMetadata member?  I don't see any other usage for this flag
in this CL.

If you have some reason to keep it in FeatureMetadata, probably you need to
set it false in FeatureMetadata::clear() below.

[rune, 2016/02/15 07:27:04]

It's passed recursively down to selectorList below. Note that
isSiblingSelector() also covers :nth-*, :only-*, etc. So for instance for:

:not(:nth-child(even)) ::content :not(:only-child) {}

the flag needs to be passed to collectFeaturesFromSelector() for the inner
selectors of :not and be different in those cases. But, I could use a bool
parameter to collectFeaturesFromSelector instead?
Good catch, but it probably makes more sense to add a bool parameter to
collectFeaturesFromSelector instead as the flag is not used after the
collection. wdyt?

[rune, 2016/02/15 08:12:07]

Done.

[kochi, 2016/02/15 10:10:56]

Thanks for explanation.  I overlooked that it is passed down recursively.

As you already resolved, it looks like keeping the flag in FeatureMetadata makes
sense.

             metadata.foundSiblingSelector = true;
 
-        const CSSSelectorList* selectorList = current->selectorList();
-        if (!selectorList)
-            continue;
-
-        for (const CSSSelector* subSelector = selectorList->first(); subSelector; subSelector = CSSSelectorList::next(*subSelector))
-            collectFeaturesFromSelector(*subSelector, metadata);
+        if (const CSSSelectorList* selectorList = current->selectorList()) {
+            for (const CSSSelector* subSelector = selectorList->first(); subSelector; subSelector = CSSSelectorList::next(*subSelector))
+                collectFeaturesFromSelector(*subSelector, metadata);
+        }
+        if (current->relationIsAffectedByPseudoContent())
+            metadata.foundInsertionPointCrossing = true;
     }
 
     ASSERT(!maxDirectAdjacentSelectors);