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
@@ -395,52 +395,85 @@
   return pattern.selectors.some(s => s.dependsOnDOM);
 }
 
 function patternDependsOnStylesAndDOM(pattern)
 {
   return pattern.selectors.some(s => s.dependsOnStyles && s.dependsOnDOM);
 }
 
-function filterPatterns(patterns, {stylesheets, mutations})
-{
-  if (stylesheets && !mutations)
-    return patterns.filter(patternDependsOnStyles);
-
-  if (!stylesheets && mutations)
-    return patterns.filter(patternDependsOnDOM);
-
-  return patterns.slice();
-}
-
-function shouldObserveAttributes(patterns)
+function patternMaybeDependsOnAttributes(pattern)
 {
   // Observe changes to attributes if either there's a plain selector that
   // looks like an ID selector, class selector, or attribute selector in one of
   // the patterns (e.g. "a[href='https://example.com/']")
   // or there's a properties selector nested inside a has selector
   // (e.g. "div:-abp-has(:-abp-properties(color: blue))")
-  return patterns.some(
-    pattern => pattern.selectors.some(
-      selector => selector.maybeDependsOnAttributes ||
-                  (selector instanceof HasSelector &&
-                   selector.dependsOnStyles)
-    )
+  return pattern.selectors.some(
+    selector => selector.maybeDependsOnAttributes ||
+                (selector instanceof HasSelector &&
+                 selector.dependsOnStyles)
   );
 }
 
+function patternDependsOnCharacterData(pattern)
+{
+  // Observe changes to character data only if there's a contains selector in
+  // one of the patterns.
+  return pattern.selectors.some(selector => selector.dependsOnCharacterData);
+}
+
+function patternMatchesMutationTypes(pattern, mutationTypes)
+{
+  return mutationTypes.has("childList") ||
+         (mutationTypes.has("attributes") &&
+          patternMaybeDependsOnAttributes(pattern)) ||
+         (mutationTypes.has("characterData") &&
+          patternDependsOnCharacterData(pattern));
+}
+
+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 && patternDependsOnStyles(pattern)) ||
+               (mutations && patternDependsOnDOM(pattern) &&
+                patternMatchesMutationTypes(pattern, mutationTypes))
+  );
+}
+
+function shouldObserveAttributes(patterns)
+{
+  return patterns.some(patternMaybeDependsOnAttributes);
+}
+
 function shouldObserveCharacterData(patterns)
 {
-  // Observe changes to character data only if there's a contains selector in
-  // one of the patterns.
-  return patterns.some(
-    pattern => pattern.selectors.some(
-      selector => selector.dependsOnCharacterData
-    )
-  );
+  return patterns.some(patternDependsOnCharacterData);
 }
 
 function ElemHideEmulation(addSelectorsFunc, hideElemsFunc)
 {
   this.document = document;
   this.addSelectorsFunc = addSelectorsFunc;
   this.hideElemsFunc = hideElemsFunc;
   this.observer = new MutationObserver(this.observe.bind(this));