Restore the browser's scroll position on server-initiated reload.

(imported from commit 934659c0f1a2c014cfd2811fe49ea31abad262b2)

static/js/message_list.js

@@ -143,6 +143,7 @@ MessageList.prototype = {
     select_id: function MessageList_select_id(id, opts) {
         opts = _.extend({
                 then_scroll: false,
+                target_scroll_offset: undefined,
                 use_closest: false,
                 empty_ok: false,
                 mark_read: true,

static/js/message_store.js

@@ -490,7 +490,8 @@ $(function () {
         // a stream from the home before already
         if (home_msg_list.selected_id() === -1 && !home_msg_list.empty()) {
             home_msg_list.select_id(page_params.initial_pointer,
-                {then_scroll: true, use_closest: true});
+                                    {then_scroll: true, use_closest: true,
+                                     target_scroll_offset: page_params.initial_offset});
         }
 
         // catch the user up

static/js/reload.js

@@ -37,6 +37,10 @@ function preserve_state(send_after_reload) {
     if (pointer !== -1) {
         url += "+pointer=" + pointer;
     }
+    var row = current_msg_list.selected_row();
+    if (row.length > 0) {
+        url += "+offset=" + row.offset().top;
+    }
 
     var oldhash = window.location.hash;
     if (oldhash.length !== 0 && oldhash[0] === '#') {
@@ -88,6 +92,10 @@ $(function () {
         page_params.orig_initial_pointer = page_params.initial_pointer;
         page_params.initial_pointer = pointer;
     }
+    var offset = parseInt(vars.offset, 10);
+    if (offset) {
+        page_params.initial_offset = offset;
+    }
 
     activity.new_user_input = false;
     hashchange.changehash(vars.oldhash);

static/js/ui.js

@@ -1469,13 +1469,16 @@ $(function () {
                     found_in_dom: row_from_dom.length
                 });
             }
-
+            if (event.target_scroll_offset !== undefined) {
+                viewport.set_message_offset(event.target_scroll_offset);
+            } else {
                 // Scroll to place the message within the current view;
                 // but if this is the initial placement of the pointer,
                 // just place it in the very center
                 recenter_view(row, {from_scroll: event.from_scroll,
                                     force_center: event.previously_selected === -1});
             }
+        }
     });
 
     $("#main_div").on("mouseenter", ".message_time", function (e) {