Bug 1382248 - avoid loading autoscroll.png at startup. r=florian

MozReview-Commit-ID: Ks6uw1tjbkG

browser/base/content/test/performance/browser_startup_images.js

@@ -64,10 +64,6 @@ const whitelist = [
     file: "chrome://browser/skin/tracking-protection-16.svg#enabled",
     platforms: ["linux", "win", "macosx"],
   },
-  {
-    file: "chrome://global/skin/icons/autoscroll.png",
-    platforms: ["linux", "win", "macosx"],
-  },
 
   {
     file: "chrome://browser/skin/tabbrowser/tabDragIndicator.png",

toolkit/content/widgets/browser.xml

@@ -1268,6 +1268,7 @@
             // events can be handled by browser-content.js.
             popup.setAttribute("mousethrough", "always");
             popup.setAttribute("rolluponmousewheel", "true");
+            popup.setAttribute("hidden", "true");
             return popup;
           ]]>
         </body>
@@ -1298,6 +1299,7 @@
               this._autoScrollPopup.setAttribute("translucent", /Win|Mac/.test(navigator.platform));
             }
 
+            this._autoScrollPopup.removeAttribute("hidden");
             this._autoScrollPopup.setAttribute("noautofocus", "true");
             this._autoScrollPopup.setAttribute("scrolldir", scrolldir);
             this._autoScrollPopup.addEventListener("popuphidden", this, true);