Index: lib/filterStorage.js
===================================================================
--- a/lib/filterStorage.js
+++ b/lib/filterStorage.js
@@ -123,16 +123,43 @@
    * @type {number}
    */
   get subscriptionCount()
   {
     return this.knownSubscriptions.size;
   }
 
   /**
+   * Yields subscriptions in the storage to which the given filter belongs.
+   * @param {Filter} filter
+   * @yields {Subscription}
+   */
+  *subscriptionsForFilter(filter)
+  {
+    if (!filter.subscriptionsAdded)
+      addSubscriptionsToFilter(filter);
+
+    yield* filter.subscriptions();
+  }
+
+  /**
+   * Returns the number of subscriptions in the storage to which the given
+   * filter belongs.
+   * @param {Filter} filter
+   * @return {number}
+   */
+  getSubscriptionCountForFilter(filter)
+  {
+    if (!filter.subscriptionsAdded)
+      addSubscriptionsToFilter(filter);
+
+    return filter.subscriptionCount;
+  }
+
+  /**
    * Finds the filter group that a filter should be added to by default. Will
    * return <code>null</code> if this group doesn't exist yet.
    * @param {Filter} filter
    * @returns {?SpecialSubscription}
    */
   getGroupForFilter(filter)
   {
     let generalSubscription = null;
@@ -221,17 +248,17 @@
    * @param {number} [position] The position within the subscription at which
    *   the filter should be added. If not specified, the filter is added at the
    *   end of the subscription.
    */
   addFilter(filter, subscription, position)
   {
     if (!subscription)
     {
-      for (let currentSubscription of filter.subscriptions())
+      for (let currentSubscription of this.subscriptionsForFilter(filter))
       {
         if (currentSubscription instanceof SpecialSubscription &&
             !currentSubscription.disabled)
         {
           return;   // No need to add
         }
       }
       subscription = this.getGroupForFilter(filter);
@@ -260,17 +287,17 @@
    *   removed from all subscriptions.
    * @param {number} [position] The position within the subscription at which
    *   the filter should be removed. If not specified, all instances of the
    *   filter will be removed.
    */
   removeFilter(filter, subscription, position)
   {
     let subscriptions = (
-      subscription ? [subscription] : filter.subscriptions()
+      subscription ? [subscription] : this.subscriptionsForFilter(filter)
     );
     for (let currentSubscription of subscriptions)
     {
       if (currentSubscription instanceof SpecialSubscription)
       {
         let positions = [];
         if (typeof position == "undefined")
         {
@@ -657,16 +684,32 @@
  * Reads the user's filters from disk, manages them in memory, and writes them
  * back to disk.
  */
 let filterStorage = new FilterStorage();
 
 exports.filterStorage = filterStorage;
 
 /**
+ * Adds to a filter any subscriptions in the storage to which the filter
+ * belongs.
+ * @param {Filter} filter
+ */
+function addSubscriptionsToFilter(filter)
+{
+  for (let subscription of filterStorage.subscriptions())
+  {
+    if (subscription.hasFilter(filter))
+      filter.addSubscription(subscription);
+  }
+
+  filter.subscriptionsAdded = true;
+}
+
+/**
  * Connects a subscription to its filters without any notifications.
  * @param {Subscription} subscription The subscription that should be
  *   connected to its filters.
  * @param {?Array.<Filter>} [filters] A list of filters to which the
  *   subscription should be connected. If this is not given, the subscription
  *   is connected to its own filters.
  */
 function connectSubscriptionFilters(subscription, filters)