Do not do finds within the event method. Decreases likelihood of hitting bug in…

Do not do finds within the event method. Decreases likelihood of hitting bug in older versions of Chrome.
https://edx.lighthouseapp.com/projects/102637/tickets/153

common/lib/xmodule/xmodule/js/src/html/edit.coffee

@@ -10,7 +10,8 @@ class @HTMLEditingDescriptor
       lineWrapping: true
     })
 
-    $(@advanced_editor.getWrapperElement()).addClass(HTMLEditingDescriptor.isInactiveClass)
+    @$advancedEditorWrapper = $(@advanced_editor.getWrapperElement())
+    @$advancedEditorWrapper.addClass(HTMLEditingDescriptor.isInactiveClass)
 
 #   This is a workaround for the fact that tinyMCE's baseURL property is not getting correctly set on AWS
 #   instances (like sandbox). It is not necessary to explicitly set baseURL when running locally.
@@ -43,16 +44,21 @@ class @HTMLEditingDescriptor
       theme_advanced_blockformats : "p,pre,h1,h2,h3",
       width: '100%',
       height: '400px',
-      setup : HTMLEditingDescriptor.setupTinyMCE,
+      setup : @setupTinyMCE,
       # Cannot get access to tinyMCE Editor instance (for focusing) until after it is rendered.
       # The tinyMCE callback passes in the editor as a paramter.
       init_instance_callback: @focusVisualEditor
     })
 
     @showingVisualEditor = true
-    @element.on('click', '.editor-tabs .tab', this, @onSwitchEditor)
+    # Doing these find operations within onSwitchEditor leads to sporadic failures on Chrome (version 20 and older).
+    $element = $(element)
+    @$htmlTab = $element.find('.html-tab')
+    @$visualTab = $element.find('.visual-tab')
 
-  @setupTinyMCE: (ed) ->
+    @element.on('click', '.editor-tabs .tab', @onSwitchEditor)
+
+  setupTinyMCE: (ed) =>
     ed.addButton('wrapAsCode', {
       title : 'Code',
       image : '/static/images/ico-tinymce-code.png',
@@ -67,22 +73,23 @@ class @HTMLEditingDescriptor
       command.setActive('wrapAsCode', e.nodeName == 'CODE')
     )
 
-  onSwitchEditor: (e)=>
-    e.preventDefault();
+    @visualEditor = ed
 
-    if not $(e.currentTarget).hasClass('current')
-      element = e.data.element
+  onSwitchEditor: (e) =>
+    e.preventDefault();
 
-      $(e.currentTarget).addClass('current')
-      $(element).find('table.mceToolbar').toggleClass(HTMLEditingDescriptor.isInactiveClass)
-      $(@advanced_editor.getWrapperElement()).toggleClass(HTMLEditingDescriptor.isInactiveClass)
+    $currentTarget = $(e.currentTarget)
+    if not $currentTarget.hasClass('current')
+      $currentTarget.addClass('current')
+      @$mceToolbar.toggleClass(HTMLEditingDescriptor.isInactiveClass)
+      @$advancedEditorWrapper.toggleClass(HTMLEditingDescriptor.isInactiveClass)
 
-      visualEditor = @getVisualEditor(element)
-      if $(e.currentTarget).attr('data-tab') is 'visual'
-        $(element).find('.html-tab').removeClass('current')
+      visualEditor = @getVisualEditor()
+      if $currentTarget.data('tab') is 'visual'
+        @$htmlTab.removeClass('current')
         @showVisualEditor(visualEditor)
       else
-        $(element).find('.visual-tab').removeClass('current')
+        @$visualTab.removeClass('current')
         @showAdvancedEditor(visualEditor)
 
   # Show the Advanced (codemirror) Editor. Pulled out as a helper method for unit testing.
@@ -104,20 +111,24 @@ class @HTMLEditingDescriptor
     @focusVisualEditor(visualEditor)
     @showingVisualEditor = true
 
-  focusVisualEditor: (visualEditor) ->
+  focusVisualEditor: (visualEditor) =>
     visualEditor.focus()
+    if not @$mceToolbar?
+      @$mceToolbar = $(@element).find('table.mceToolbar')
 
-  getVisualEditor: (element) ->
+  getVisualEditor: () ->
     ###
     Returns the instance of TinyMCE.
     This is different from the textarea that exists in the HTML template (@tiny_mce_textarea.
+
+    Pulled out as a helper method for unit test.
     ###
-    return tinyMCE.get($(element).find('.tiny-mce').attr('id'))
+    return @visualEditor
 
   save: ->
     @element.off('click', '.editor-tabs .tab', @onSwitchEditor)
     text = @advanced_editor.getValue()
-    visualEditor = @getVisualEditor(@element)
+    visualEditor = @getVisualEditor()
     if @showingVisualEditor and visualEditor.isDirty()
       text = visualEditor.getContent({no_events: 1})
     data: text