Issue 6423 - Observe only relevant mutation types

lib/content/elemHideEmulation.js

Index: lib/content/elemHideEmulation.js
===================================================================
--- a/lib/content/elemHideEmulation.js
+++ b/lib/content/elemHideEmulation.js
@@ -190,16 +190,17 @@
   // document.querySelectorAll() call which didn't produce any results, make
   // sure there is at least one point where execution can pause.
   yield null;
 }
 
 function PlainSelector(selector)
 {
   this._selector = selector;
+  this.maybeDependsOnAttributes = /[#.]|\[.+\]/.test(selector);
 }
 
 PlainSelector.prototype = {
   /**
    * Generator function returning a pair of selector
    * string and subtree.
    * @param {string} prefix the prefix for the selector.
    * @param {Node} subtree the subtree we work on.
@@ -221,16 +222,30 @@
 HasSelector.prototype = {
   requiresHiding: true,
 
   get dependsOnStyles()
   {
     return this._innerSelectors.some(selector => selector.dependsOnStyles);
   },
 
+  get dependsOnCharacterData()
+  {
+    return this._innerSelectors.some(
+      selector => selector.dependsOnCharacterData
+    );
+  },
+
+  get maybeDependsOnAttributes()
+  {
+    return this._innerSelectors.some(
+      selector => selector.maybeDependsOnAttributes
+    );
+  },
+
   *getSelectors(prefix, subtree, styles)
   {
     for (let element of this.getElements(prefix, subtree, styles))
       yield [makeSelector(element, ""), element];
   },
 
   /**
    * Generator function returning selected elements.
@@ -263,16 +278,17 @@
 
 function ContainsSelector(textContent)
 {
   this._text = textContent;
 }
 
 ContainsSelector.prototype = {
   requiresHiding: true,
+  dependsOnCharacterData: true,
 
   *getSelectors(prefix, subtree, stylesheet)
   {
     for (let element of this.getElements(prefix, subtree, stylesheet))
       yield [makeSelector(element, ""), subtree];
   },
 
   *getElements(prefix, subtree, stylesheet)
@@ -339,16 +355,43 @@
 };
 
 function isSelectorHidingOnlyPattern(pattern)
 {
   return pattern.selectors.some(s => s.preferHideWithSelector) &&
     !pattern.selectors.some(s => s.requiresHiding);
 }
 
+function shouldObserveAttributes(patterns)
+{
+  // 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)
+    )
+  );
+}
+
+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
+    )
+  );
+}
+
 function ElemHideEmulation(addSelectorsFunc, hideElemsFunc)
 {
   this.document = document;
   this.addSelectorsFunc = addSelectorsFunc;
   this.hideElemsFunc = hideElemsFunc;
   this.observer = new MutationObserver(this.observe.bind(this));
 }
 
@@ -637,18 +680,18 @@
 
     if (this.patterns.length > 0)
     {
       this.queueFiltering();
       this.observer.observe(
         this.document,
         {
           childList: true,
-          attributes: true,
-          characterData: true,
+          attributes: shouldObserveAttributes(this.patterns),
+          characterData: shouldObserveCharacterData(this.patterns),
           subtree: true
         }
       );
       this.document.addEventListener("load", this.onLoad.bind(this), true);
     }
   }
 };