Cache only last response in markdown preview

app/assets/javascripts/markdown_preview.js.coffee

@@ -25,7 +25,7 @@ class @MarkdownPreview
   renderMarkdown: (text, success) ->
     return unless window.markdown_preview_path
 
-    return success(@ajaxCache[text]) if @ajaxCache[text]
+    return success(@ajaxCache.response) if text == @ajaxCache.text
 
     $.ajax
       type: 'POST'
@@ -33,7 +33,7 @@ class @MarkdownPreview
       data: { text: text }
       dataType: 'json'
       success: (response) =>
-        @ajaxCache[text] = response
+        @ajaxCache = text: text, response: response
         success(response)
 
   hideReferencedUsers: (form) ->

app/views/help/_shortcuts.html.haml

@@ -24,7 +24,11 @@
                 %td Show this dialog
               %tr
                 %td.shortcut
+                  - if browser.mac?
+                    .key ctrl ⌘ p
+                  - else
                     .key ctrl shift p
+
                 %td Toggle Markdown preview
             %tbody
               %tr