[cros] Pre-select the first user pod on login screen.

chrome/browser/resources/chromeos/login/user_pod_row.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview User pod row implementation.
  */
 
 cr.define('login', function() {
   /**
@@ skipping 641 matching lines @@
       clearTimeout(this.loadWallpaperTimeout_);
       for (var i = 0, pod; pod = this.pods[i]; ++i) {
         pod.activeRemoveButton = false;
         if (pod != podToFocus) {
           pod.classList.remove('focused');
           pod.classList.remove('faded');
           pod.reset(false);
         }
       }
 
+      var hadFocus = !!this.focusedPod_;
       this.focusedPod_ = podToFocus;
       if (podToFocus) {
         podToFocus.classList.remove('faded');
         podToFocus.classList.add('focused');
         podToFocus.reset(true);  // Reset and give focus.
         this.scrollPodIntoView(podToFocus);
-        this.loadWallpaperTimeout_ = window.setTimeout(function() {
-          chrome.send('userSelectedDelayed', [podToFocus.user.username]);
-        }, WALLPAPER_LOAD_DELAY_MS);
+        // Load wallpaper immediately if there no pod was focused previously,
+        // otherwise delay loading to let user tab through pods without lag.
+        if (hadFocus) {
+          this.loadWallpaperTimeout_ = window.setTimeout(
+              this.loadWallpaper_.bind(this), WALLPAPER_LOAD_DELAY_MS);
+        } else {
+          this.loadWallpaper_();
+        }
+      } else {
+        // TODO(ivankr): reset wallpaper.
       }
     },
 
+    loadWallpaper_: function() {
+      if (this.focusedPod_)
+        chrome.send('userSelectedDelayed', [this.focusedPod_.user.username]);
+    },
+
     /**
      * Returns the currently activated pod.
      * @type {UserPod}
      */
     get activatedPod() {
       return this.activatedPod_;
     },
     set activatedPod(pod) {
       if (pod && pod.activate())
         this.activatedPod_ = pod;
@@ skipping 187 matching lines @@
           chrome.send('loginVisible');
         });
       }
     }
   };
 
   return {
     PodRow: PodRow
   };
 });