Remove ForVTT codepath in SelectorMatcher

Having a separate copy-pasted codepath for selector matching is not
acceptable for maintainability reasons. Instead we can lazily create
an actual Element to represent the almost-featureless element
described by the spec [1], and use this as the originating element
during selector matching. The element has no ID, no classes, no
attributes, no tag name, no namespace, no parent, no siblings; so it
should be pretty hard to match against it.

Longer term it would be nice to make the originating element actually
featureless [2].

Bug: 1137349
TEST=external/wpt/webvtt/rendering/cues-with-video/processing-model/support/embedded_style_selectors.html

[1] https://w3c.github.io/webvtt/#obtaining-css-boxes
[2] https://github.com/w3c/webvtt/issues/477

Change-Id: I06d618546893be3942ea5406507a01bc508340d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462274
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816299}

third_party/blink/renderer/core/css/element_rule_collector.cc

@@ -150,7 +150,7 @@ void ElementRuleCollector::CollectMatchingRulesForList(
       &context_.GetElement(), SelectorChecker::kVisitedMatchEnabled);
   context.scope = match_request.scope;
   context.pseudo_id = pseudo_style_request_.pseudo_id;
-  context.is_from_vtt = match_request.is_from_vtt;
+  context.vtt_originating_element = match_request.vtt_originating_element;
 
   unsigned rejected = 0;
   unsigned fast_rejected = 0;

third_party/blink/renderer/core/css/resolver/match_request.h

@@ -42,12 +42,12 @@ class MatchRequest {
                const ContainerNode* scope = nullptr,
                const CSSStyleSheet* css_sheet = nullptr,
                unsigned style_sheet_index = 0,
-               bool is_from_vtt = false)
+               Element* vtt_originating_element = nullptr)
       : rule_set(rule_set),
         scope(scope),
         style_sheet(css_sheet),
         style_sheet_index(style_sheet_index),
-        is_from_vtt(is_from_vtt) {
+        vtt_originating_element(vtt_originating_element) {
     // Now that we're about to read from the RuleSet, we're done adding more
     // rules to the set and we should make sure it's compacted.
     rule_set->CompactRulesIfNeeded();
@@ -57,7 +57,10 @@ class MatchRequest {
   const ContainerNode* scope;
   const CSSStyleSheet* style_sheet;
   const unsigned style_sheet_index;
-  bool is_from_vtt;
+  // For WebVTT STYLE blocks, this is set to the featureless-like Element
+  // described by the spec:
+  // https://w3c.github.io/webvtt/#obtaining-css-boxes
+  Element* vtt_originating_element;
 };
 
 }  // namespace blink

third_party/blink/renderer/core/css/resolver/style_resolver.cc

@@ -409,12 +409,12 @@ static void MatchVTTRules(const Element& element,
     int style_sheet_index = 0;
     collector.ClearMatchedRules();
     for (CSSStyleSheet* style : styles) {
-      RuleSet* rule_set =
-          element.GetDocument().GetStyleEngine().RuleSetForSheet(*style);
+      StyleEngine& style_engine = element.GetDocument().GetStyleEngine();
+      RuleSet* rule_set = style_engine.RuleSetForSheet(*style);
       if (rule_set) {
-        collector.CollectMatchingRules(
-            MatchRequest(rule_set, nullptr /* scope */, style,
-                         style_sheet_index, true /* is_from_webvtt */));
+        collector.CollectMatchingRules(MatchRequest(
+            rule_set, nullptr /* scope */, style, style_sheet_index,
+            style_engine.EnsureVTTOriginatingElement()));
         style_sheet_index++;
       }
     }

third_party/blink/renderer/core/css/selector_checker.h

@@ -125,7 +125,7 @@ class SelectorChecker {
     bool has_scrollbar_pseudo = false;
     bool has_selection_pseudo = false;
     bool treat_shadow_host_as_normal_scope = false;
-    bool is_from_vtt = false;
+    Element* vtt_originating_element = nullptr;
     bool in_nested_complex_selector = false;
   };
 
@@ -155,7 +155,6 @@ class SelectorChecker {
   // to by the context are a match. Delegates most of the work to the Check*
   // methods below.
   bool CheckOne(const SelectorCheckingContext&, MatchResult&) const;
-  bool CheckOneForVTT(const SelectorCheckingContext&, MatchResult&) const;
 
   enum MatchStatus {
     kSelectorMatches,
@@ -182,35 +181,22 @@ class SelectorChecker {
   // to try (e.g. same element, parent, sibling) depends on the combinators in
   // the selectors.
   MatchStatus MatchSelector(const SelectorCheckingContext&, MatchResult&) const;
-  MatchStatus MatchSelectorForVTT(const SelectorCheckingContext&,
-                                  MatchResult&) const;
   MatchStatus MatchForSubSelector(const SelectorCheckingContext&,
                                   MatchResult&) const;
-  MatchStatus MatchForSubSelectorForVTT(const SelectorCheckingContext&,
-                                        MatchResult&) const;
   MatchStatus MatchForRelation(const SelectorCheckingContext&,
                                MatchResult&) const;
-  MatchStatus MatchForRelationForVTT(const SelectorCheckingContext&,
-                                     MatchResult&) const;
   MatchStatus MatchForPseudoContent(const SelectorCheckingContext&,
                                     const Element&,
                                     MatchResult&) const;
   MatchStatus MatchForPseudoShadow(const SelectorCheckingContext&,
                                    const ContainerNode*,
                                    MatchResult&) const;
-  bool MatchVTTBlockSelector(const SelectorCheckingContext& context,
-                             MatchResult& result) const;
   bool CheckPseudoClass(const SelectorCheckingContext&, MatchResult&) const;
-  bool CheckPseudoClassForVTT(const SelectorCheckingContext&,
-                              MatchResult&) const;
   bool CheckPseudoElement(const SelectorCheckingContext&, MatchResult&) const;
-  bool CheckPseudoElementForVTT(const SelectorCheckingContext&,
-                                MatchResult&) const;
   bool CheckScrollbarPseudoClass(const SelectorCheckingContext&,
                                  MatchResult&) const;
   bool CheckPseudoHost(const SelectorCheckingContext&, MatchResult&) const;
   bool CheckPseudoNot(const SelectorCheckingContext&, MatchResult&) const;
-  bool CheckPseudoNotForVTT(const SelectorCheckingContext&, MatchResult&) const;
 
   ComputedStyle* element_style_;
   CustomScrollbar* scrollbar_;

third_party/blink/renderer/core/css/style_engine.cc

@@ -470,6 +470,14 @@ void StyleEngine::RemoveTextTrack(TextTrack* text_track) {
   text_tracks_.erase(text_track);
 }
 
+Element* StyleEngine::EnsureVTTOriginatingElement() {
+  if (!vtt_originating_element_) {
+    vtt_originating_element_ = MakeGarbageCollected<Element>(
+        QualifiedName(g_null_atom, g_empty_atom, g_empty_atom), document_);
+  }
+  return vtt_originating_element_;
+}
+
 void StyleEngine::MediaQueryAffectingValueChanged(
     HeapHashSet<Member<TextTrack>>& text_tracks,
     MediaValueChange change) {
@@ -2344,6 +2352,7 @@ void StyleEngine::Trace(Visitor* visitor) const {
   visitor->Trace(tracker_);
   visitor->Trace(meta_color_scheme_);
   visitor->Trace(text_tracks_);
+  visitor->Trace(vtt_originating_element_);
   FontSelectorClient::Trace(visitor);
 }
 

third_party/blink/renderer/core/css/style_engine.h

@@ -133,6 +133,14 @@ class CORE_EXPORT StyleEngine final : public GarbageCollected<StyleEngine>,
   void AddTextTrack(TextTrack*);
   void RemoveTextTrack(TextTrack*);
 
+  // An Element with no tag name, IDs, classes (etc), as described by the
+  // WebVTT spec:
+  // https://w3c.github.io/webvtt/#obtaining-css-boxes
+  //
+  // TODO(https://github.com/w3c/webvtt/issues/477): Make originating element
+  // actually featureless.
+  Element* EnsureVTTOriginatingElement();
+
   const ActiveStyleSheetVector ActiveStyleSheetsForInspector();
 
   bool NeedsActiveStyleUpdate() const;
@@ -661,6 +669,7 @@ class CORE_EXPORT StyleEngine final : public GarbageCollected<StyleEngine>,
   friend class WhitespaceAttacherTest;
 
   HeapHashSet<Member<TextTrack>> text_tracks_;
+  Member<Element> vtt_originating_element_;
 };
 
 }  // namespace blink