message_list: Track date better in floating recipient bar.

We now use the date from the first message
that's at least partially visible on the screen.

static/js/floating_recipient_bar.js

@@ -5,6 +5,48 @@ var exports = {};
 
 var is_floating_recipient_bar_showing = false;
 
+exports.first_visible_message = function (bar) {
+    var messages = bar.children('.message_row');
+    var frb_bottom = exports.frb_bottom();
+
+    for (var i = 0; i < messages.length; i += 1) {
+        var message = $(messages[i]);
+        var message_bottom = message.offset().top + message.safeOuterHeight();
+
+        if (message_bottom > frb_bottom) {
+            return message;
+        }
+
+        message = message.next('.message_row');
+    }
+};
+
+exports.copy_message_date = function (source_recipient_bar) {
+    var message_row = exports.first_visible_message(source_recipient_bar);
+
+    if (!message_row || !message_row.length) {
+        return;
+    }
+
+    var msg_id = rows.id(message_row);
+
+    if (msg_id === undefined) {
+        return;
+    }
+
+    var message = message_store.get(msg_id);
+
+    if (!message) {
+        return;
+    }
+
+    var time = new XDate(message.timestamp * 1000);
+    var today = new XDate();
+    var rendered_date = timerender.render_date(time, undefined, today)[0].outerHTML;
+
+    $('#floating_recipient_bar').find('.recipient_row_date').html(rendered_date);
+};
+
 exports.frb_bottom = function () {
     var bar = $("#floating_recipient_bar");
     var bar_top = bar.offset().top;
@@ -71,6 +113,8 @@ function replace_floating_recipient_bar(source_recipient_bar) {
         new_label.toggleClass('message-fade', source_recipient_bar.hasClass('message-fade'));
         old_source = source_recipient_bar;
     }
+
+    exports.copy_message_date(source_recipient_bar);
     show_floating_recipient_bar();
 }