Issue 6504 - Remove useInlineStyles for now

lib/content/elemHideEmulation.js

Index: lib/content/elemHideEmulation.js
===================================================================
--- a/lib/content/elemHideEmulation.js
+++ b/lib/content/elemHideEmulation.js
@@ -599,16 +599,23 @@
 }
 
 function ElemHideEmulation(addSelectorsFunc, hideElemsFunc)
 {
   this.document = document;
   this.addSelectorsFunc = addSelectorsFunc;
   this.hideElemsFunc = hideElemsFunc;
   this.observer = new MutationObserver(this.observe.bind(this));
+
+  // This flag is supposed to indicate whether we should prefer setting the
+  // style attribute of the element over using CSS selectors. It is set by the
+  // web extension if the tabs.removeCSS API is available (Firefox 53+). But
+  // because of issues with performance (see #6422), we are ignoring the value
+  // of this flag for now; we always set the style attribute.
+  // https://issues.adblockplus.org/ticket/6504#comment:9
   this.useInlineStyles = true;
 }
 
 ElemHideEmulation.prototype = {
   isSameOrigin(stylesheet)
   {
     try
     {
@@ -801,28 +808,20 @@
 
         generator = evaluate(pattern.selectors, 0, "",
                              this.document, cssStyles, evaluationTargets);
       }
       for (let selector of generator)
       {
         if (selector != null)
         {
-          if (!this.useInlineStyles)
+          for (let element of this.document.querySelectorAll(selector))
           {
-            selectors.push(selector);
-            selectorFilters.push(pattern.text);
-          }
-          else
-          {
-            for (let element of this.document.querySelectorAll(selector))
-            {
-              elements.push(element);
-              elementFilters.push(pattern.text);
-            }
+            elements.push(element);
+            elementFilters.push(pattern.text);
           }
         }
         if (performance.now() - cycleStart > MAX_SYNCHRONOUS_PROCESSING_TIME)
         {
           setTimeout(processPatterns, 0);
           return;
         }
       }