List views persist sroll offset by percentage

Fixes #188

js/views/conversation_view.js

@@ -45,7 +45,8 @@
             });
 
             this.view = new Whisper.MessageListView({
-                collection: this.model.messageCollection
+                collection: this.model.messageCollection,
+                window: this.appWindow.contentWindow
             });
             this.$('.discussion-container').append(this.view.el);
             this.view.render();

js/views/list_view.js

@@ -24,9 +24,31 @@
     Whisper.ListView = Backbone.View.extend({
         tagName: 'ul',
         itemView: Backbone.View,
-        initialize: function() {
+        initialize: function(options) {
             this.listenTo(this.collection, 'add', this.addOne);
             this.listenTo(this.collection, 'reset', this.addAll);
+
+            if (options.window) {
+                var $window = this.$(options.window);
+                $window.scroll(this.onScroll.bind(this));
+                $window.resize(this.onResize.bind(this));
+            }
+        },
+
+        onResize: function(e) {
+            this.resizing = true;
+            clearTimeout(this.resizeTimer);
+            resizeTimer = setTimeout(function() {
+                resizing = false;
+            }, 500);
+            this.$el.scrollTop(this.scrollPercent * this.$el.height());
+        },
+
+        onScroll: function(e) {
+            if (!this.resizing) {
+                this.scrollTop = this.$el.scrollTop();
+                this.scrollPercent = this.scrollTop / this.$el.height();
+            }
         },
 
         addOne: function(model) {