Show "displays" section in the options page even for 1 display. It happens in case of mirroring.

chrome/browser/resources/options2/chromeos/display_options.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.
 
 cr.define('options', function() {
   var OptionsPage = options.OptionsPage;
 
   // The scale ratio of the display rectangle to its original size.
   /** @const */ var VISUAL_SCALE = 1 / 10;
 
@@ skipping 208 matching lines @@
       this.displays_view_.onmousedown = this.onMouseDown_.bind(this);
       this.displays_view_.onmouseup = this.onMouseUp_.bind(this);
       displays_view_host.appendChild(this.displays_view_);
     },
 
     /**
      * Lays out the display rectangles for mirroring.
      * @private
      */
     layoutMirroringDisplays_: function() {
+      // Offset pixels for secondary display rectangles.
+      /** @const */ var MIRRORING_OFFSET_PIXELS = 2;
+
       // The width/height should be same as the primary display:
       var width = this.displays_[0].width * VISUAL_SCALE;
       var height = this.displays_[0].height * VISUAL_SCALE;
 
+      // Always show two displays because there must be two displays when
+      // the display_options is enabled.  Don't rely on displays_.length because
+      // there is only one display from chrome's perspective in mirror mode.
+      var num_displays = Math.max(2, this.displays_.length);

[James Hawkins, 2012/07/30 03:14:35]

Optional nit: Pull this two into a const, e.g., MIN_NUM_DISPLAYS or something.

[Jun Mukai, 2012/07/30 07:58:41]

Done.

+
       this.displays_view_.style.height =
-        height + this.displays_.length * 2 + 'px';
+        height + num_displays * MIRRORING_OFFSET_PIXELS + 'px';
 
-      for (var i = 0; i < this.displays_.length; i++) {
+      for (var i = 0; i < num_displays; i++) {
         var div = document.createElement('div');
-        this.displays_[i].div = div;
         div.className = 'displays-display';
-        div.style.top = i * 2 + 'px';
-        div.style.left = i * 2 + 'px';
+        div.style.top = i * MIRRORING_OFFSET_PIXELS + 'px';
+        div.style.left = i * MIRRORING_OFFSET_PIXELS + 'px';
         div.style.width = width + 'px';
         div.style.height = height + 'px';
         div.style.zIndex = i;
-        if (i == this.displays_.length - 1)
+        if (i == num_displays - 1)
           div.className += ' displays-primary';
         this.displays_view_.appendChild(div);
       }
     },
 
     /**
      * Layouts the display rectangles according to the current layout_.
      * @private
      */
     layoutDisplays_: function() {
@@ skipping 67 matching lines @@
           loadTimeData.getString(
               this.mirroring_ ? 'stopMirroring' : 'startMirroring');
 
       // Focus to the first display next to the primary one when |displays| list
       // is updated.
       if (this.displays_.length != displays.length)
         this.focused_index_ = 1;
 
       this.displays_ = displays;
 
-      if (this.displays_.length <= 1)
-        return;
-
       this.resetDisplaysView_();
       if (this.mirroring_)
         this.layoutMirroringDisplays_();
       else
         this.layoutDisplays_();
     },
   };
 
   DisplayOptions.setDisplayInfo = function(mirroring, displays, layout) {
     DisplayOptions.getInstance().onDisplayChanged_(mirroring, displays, layout);
   };
 
   // Export
   return {
     DisplayOptions: DisplayOptions
   };
 });