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
@@ -297,16 +297,44 @@
 };
 
 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 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 instanceof PlainSelector &&
+                   /\[.+\]/.test(selector._selector))||

[Manish Jethani, 2017/10/22 22:58:13]

This check for attribute selector syntax can give false positives in very rare
cases, but it doesn't matter since this is just an optimization anyway.

+                  (pattern.selectors[0] instanceof HasSelector &&

[Manish Jethani, 2017/10/22 22:58:13]

I realize that the second check is essentially
pattern.selectors[0].dependsOnStyle. I think this logic generally should be
moved into the respective classes.

[Manish Jethani, 2017/10/23 00:12:09]

Done.

+                   selector instanceof PropsSelector)
+    )
+  );
+}
+
+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 instanceof ContainsSelector
+    )
+  );
+}
+
 function ElemHideEmulation(addSelectorsFunc, hideElemsFunc)
 {
   this.document = document;
   this.addSelectorsFunc = addSelectorsFunc;
   this.hideElemsFunc = hideElemsFunc;
   this.observer = new MutationObserver(this.observe.bind(this));
 }
 
@@ -582,18 +610,18 @@
 
     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);
     }
   }
 };