Issue 3879 - CSS property filters ignore third-party stylesheets in Chrome but not in Firefox

chrome/content/cssProperties.js

 function splitSelector(selector)
 {
   if (selector.indexOf(",") == -1)
     return [selector];
 
   var selectors = [];
   var start = 0;
   var level = 0;
   var sep = "";
 
@@ skipping 38 matching lines @@
     {
       var property = style.item(i);
       var value    = style.getPropertyValue(property);
       var priority = style.getPropertyPriority(property);
       styles.push(property + ": " + value + (priority ? " !" + priority : "") + ";");
     }
     styles.sort();
     return styles.join(" ");
   },
 
-  findSelectors: function(stylesheet, selectors)
+  isSameOrigin: function(stylesheet)
   {
     try

[Sebastian Noack, 2016/04/19 14:47:01]

I guess we should move this logic to a seperate function, since V8 cannot
optimize the whole function if there is any try/catch.

[Wladimir Palant, 2016/04/19 15:07:42]

Done.

     {
-      // Explicitly ignore third-party stylesheets to ensure consistent behavior
-      // between Firefox and Chrome.
-      if (stylesheet.href &&

[Sebastian Noack, 2016/04/19 14:47:01]

This check seems to be redundant as we catch the exception anyway.

[Wladimir Palant, 2016/04/19 15:07:42]

I don't really like intentionally running into exceptions but whatever.

-          new URL(stylesheet.href).origin != this.window.location.origin)
-      {

[Sebastian Noack, 2016/04/19 14:47:01]

Nit: Redundant braces.

[Wladimir Palant, 2016/04/19 15:07:42]

Not redundant, actually required for readability since the condition itself is
multi-line. But these are gone anyway now.

-        return;
-      }
+      return new URL(stylesheet.href).origin == this.window.location.origin;
     }
     catch (e)
     {
-      // An invalid URL, can be ignored.
+      // Invalid URL, assume that it is first-party.
+      return true;
     }
+  },
+
+  findSelectors: function(stylesheet, selectors)
+  {
+    // Explicitly ignore third-party stylesheets to ensure consistent behavior
+    // between Firefox and Chrome.
+    if (!this.isSameOrigin(stylesheet))
+      return;
 
     var rules = stylesheet.cssRules;
     if (!rules)
       return;
 
     for (var i = 0; i < rules.length; i++)
     {
       var rule = rules[i];
       if (rule.type != rule.STYLE_RULE)
         continue;
@@ skipping 50 matching lines @@
   apply: function()
   {
     if (this.patterns.length > 0)
     {
       var document = this.window.document;
       this.addSelectors(document.styleSheets);
       document.addEventListener("load", this.onLoad.bind(this), true);
     }
   }
 };