Issue 6437 - Filter out patterns that do not match DOM mutations

lib/content/elemHideEmulation.js

Index: lib/content/elemHideEmulation.js
===================================================================
--- a/lib/content/elemHideEmulation.js
+++ b/lib/content/elemHideEmulation.js
@@ -458,27 +458,69 @@
   get dependsOnCharacterData()
   {
     // Observe changes to character data only if there's a contains selector in
     // one of the patterns.
     return getCachedPropertyValue(
       this, "_dependsOnCharacterData",
       () => this.selectors.some(selector => selector.dependsOnCharacterData)
     );
+  },
+
+  matchesMutationTypes(mutationTypes)
+  {
+    let mutationTypeMatchMap = getCachedPropertyValue(
+      this, "_mutationTypeMatchMap",
+      () => new Map([
+        // All types of DOM-dependent patterns are affected by mutations of
+        // type "childList".
+        ["childList", true],
+        ["attributes", this.maybeDependsOnAttributes],
+        ["characterData", this.dependsOnCharacterData]
+      ])
+    );
+
+    for (let mutationType of mutationTypes)
+    {
+      if (mutationTypeMatchMap.get(mutationType))
+        return true;
+    }
+
+    return false;
   }
 };
 
+function extractMutationTypes(mutations)
+{
+  let types = new Set();
+
+  for (let mutation of mutations)
+  {
+    types.add(mutation.type);
+
+    // There are only 3 types of mutations: "attributes", "characterData", and
+    // "childList".
+    if (types.size == 3)
+      break;
+  }
+
+  return types;
+}
+
 function filterPatterns(patterns, {stylesheets, mutations})
 {
   if (!stylesheets && !mutations)
     return patterns.slice();
 
+  let mutationTypes = mutations ? extractMutationTypes(mutations) : null;
+
   return patterns.filter(
     pattern => (stylesheets && pattern.dependsOnStyles) ||
-               (mutations && pattern.dependsOnDOM)
+               (mutations && pattern.dependsOnDOM &&
+                pattern.matchesMutationTypes(mutationTypes))
   );
 }
 
 function shouldObserveAttributes(patterns)
 {
   return patterns.some(pattern => pattern.maybeDependsOnAttributes);
 }