Optimize setting data operations.

Setting local data on Chrome is not throttled, only debounce sync.
Don't make DBs save data on every page load, don't save cleaning operations if nothing's changed.
Debounce Unread.saveLastReadPost() on Chrome to every 5 seconds.

css/style.css

@@ -787,6 +787,7 @@ a.hide-announcement {
   min-width: 100%;
   overflow: hidden;
   white-space: nowrap;
+  position: relative;
   -webkit-user-select: none;
   -moz-user-select: none;
   user-select: none;

lib/$.coffee

@@ -288,8 +288,7 @@ do ->
       delete timeout[area]
     items[area] = {}
 
-  setAll = $.debounce $.SECOND, ->
-    setArea 'local'
+  setSync = $.debounce $.SECOND, ->
     setArea 'sync'
 
   $.set = (key, val) ->
@@ -300,7 +299,8 @@ do ->
     for key in $.localKeys when key of items.sync
       items.local[key] = items.sync[key]
       delete items.sync[key]
-    setAll()
+    setArea 'local'
+    setSync()
 
   $.clear = (cb) ->
     items.local = {}

src/General/DataBoard.coffee

@@ -58,15 +58,14 @@ class DataBoard
     val or defaultValue
 
   clean: ->
-    for boardID, val of @data.boards
-      @deleteIfEmpty {boardID}
-
     now = Date.now()
-    if (@data.lastChecked or 0) < now - 2 * $.HOUR
-      @data.lastChecked = now
-      for boardID of @data.boards
-        @ajaxClean boardID
+    return if (@data.lastChecked or 0) > now - 2 * $.HOUR
 
+    for boardID of @data.boards
+      @deleteIfEmpty {boardID}
+      @ajaxClean boardID if boardID of @data.boards
+
+    @data.lastChecked = now
     @save()
   ajaxClean: (boardID) ->
     $.cache "//a.4cdn.org/#{boardID}/threads.json", (e) =>
@@ -76,12 +75,14 @@ class DataBoard
       board   = @data.boards[boardID]
       threads = {}
       for page in e.target.response
-        for thread in page.threads
-          if thread.no of board
-            threads[thread.no] = board[thread.no]
-      @data.boards[boardID] = threads
-      @deleteIfEmpty {boardID}
-      @save()
+        for thread in page.threads when thread.no of board
+          threads[thread.no] = board[thread.no]
+      count = Object.keys(threads).length
+      return if count is Object.keys(board).length # Nothing changed.
+      if count
+        @set {boardID, val: threads}
+      else
+        @delete {boardID}
 
   onSync: (data) =>
     @data = data or boards: {}

src/Monitoring/Unread.coffee

@@ -139,7 +139,7 @@ Unread =
     Unread.readArray Unread.postsQuotingYou
     Unread.update() if e
 
-  saveLastReadPost: ->
+  saveLastReadPost: <% if (type === 'crx') { %>$.debounce 5 * $.SECOND,<% } %> ->
     return if Unread.thread.isDead
     Unread.db.set
       boardID:  Unread.thread.board.ID