Increase the notes polling timeout over time

A file called notes is loaded every ~15 seconds which checks for
updates to content on the page. This commit increases the polling
timeout over time (15, 30, 60, 120 seconds) and resets
it to 15 seconds if the AJAX call returns new notes

Fixes issue #13300

app/assets/javascripts/notes.js.coffee

@@ -16,10 +16,12 @@ class @Notes
     @view = view
     @noteable_url = document.URL
     @notesCountBadge ||= $(".issuable-details").find(".notes-tab .badge")
+    @base_polling_interval = 15000
+    @limit_polling_interval = 120000
 
     @cleanBinding()
     @addBinding()
-    @initRefresh()
+    @setPollingInterval()
     @setupMainTargetNoteForm()
     @initTaskList()
 
@@ -91,7 +93,7 @@ class @Notes
     clearInterval(Notes.interval)
     Notes.interval = setInterval =>
       @refresh()
-    , 15000
+    , @polling_interval
 
   refresh: ->
     if not document.hidden and document.URL.indexOf(@noteable_url) is 0
@@ -105,12 +107,28 @@ class @Notes
       success: (data) =>
         notes = data.notes
         @last_fetched_at = data.last_fetched_at
+        @setPollingInterval(data.notes.length)
         $.each notes, (i, note) =>
           if note.discussion_with_diff_html?
             @renderDiscussionNote(note)
           else
             @renderNote(note)
 
+  ###
+  Increase @polling_interval up to 120 seconds on every function call,
+  if `shouldReset` has a truthy value, 'null' or 'undefined' the variable
+  will reset to @base_polling_interval.
+
+  Note: this function is used to gradually increase the polling interval
+  if there aren't new notes coming from the server
+  ###
+  setPollingInterval: (shouldReset = true) ->
+    if shouldReset
+      @polling_interval = @base_polling_interval
+    else if @polling_interval < @limit_polling_interval
+      @polling_interval *= 2
+
+    @initRefresh()
 
   ###
   Render note in main comments area.