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

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 569 matching lines @@
         if (current->pseudoType() == CSSSelector::PseudoWindowInactive)
             metadata.usesWindowInactiveSelector = true;
         if (current->relation() == CSSSelector::DirectAdjacent) {
             maxDirectAdjacentSelectors++;
         } else if (maxDirectAdjacentSelectors
             && ((current->relation() != CSSSelector::SubSelector) || current->isLastInTagHistory())) {
             if (maxDirectAdjacentSelectors > metadata.maxDirectAdjacentSelectors)
                 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);
 }
 
 void RuleFeatureSet::FeatureMetadata::add(const FeatureMetadata& other)
 {
     usesFirstLineRules = usesFirstLineRules || other.usesFirstLineRules;
     usesWindowInactiveSelector = usesWindowInactiveSelector || other.usesWindowInactiveSelector;
     maxDirectAdjacentSelectors = std::max(maxDirectAdjacentSelectors, other.maxDirectAdjacentSelectors);
@@ skipping 121 matching lines @@
 
 DEFINE_TRACE(RuleFeatureSet)
 {
 #if ENABLE(OILPAN)
     visitor->trace(siblingRules);
     visitor->trace(uncommonAttributeRules);
 #endif
 }
 
 } // namespace blink