Do not try to recenter if you were already moving in the right direction.

Before this commit, if you try to arrow around when the selected
message is outside the pointer threshold for recentering, you get a
big jump, even if you are arrowing towards the center of the viewport.

(imported from commit 5c15d5ccccdf027a8bfa8b79bf519fccbfa971d8)

tools/jslint/check-all.js

@@ -30,7 +30,7 @@ var globals =
     + ' add_messages'
     + ' subject_dict people_dict'
     + ' keep_pointer_in_view move_pointer_at_page_top_and_bottom'
-    + ' respond_to_message recenter_view'
+    + ' respond_to_message recenter_view last_viewport_movement_direction'
     + ' scroll_to_selected disable_pointer_movement get_private_message_recipient'
     + ' load_old_messages'
     + ' at_top_of_viewport at_bottom_of_viewport within_viewport'

zephyr/static/js/hotkey.js

@@ -4,6 +4,16 @@ var exports = {};
 
 exports.in_scroll_caused_by_keypress = false;
 
+function wrap_directional_key_with_movement_direction(arrow_key_func) {
+    if (arrow_key_func === rows.next_visible) {
+        last_viewport_movement_direction = 1;
+    } else if (arrow_key_func === rows.prev_visible) {
+        last_viewport_movement_direction = -1;
+    }
+
+    return arrow_key_func;
+}
+
 var directional_hotkeys = {
     40:  rows.next_visible,  // down arrow
     106: rows.next_visible,  // 'j'
@@ -108,7 +118,8 @@ function process_hotkey(e) {
     }
 
     if (directional_hotkeys.hasOwnProperty(code)) {
-        next_row = directional_hotkeys[code](current_msg_list.selected_row());
+        next_row = wrap_directional_key_with_movement_direction(
+            directional_hotkeys[code])(current_msg_list.selected_row());
         if (next_row.length !== 0) {
             exports.in_scroll_caused_by_keypress = true;
             current_msg_list.select_id(rows.id(next_row), {then_scroll: true});

zephyr/static/js/ui.js

@@ -617,6 +617,7 @@ $(function () {
         // scroll handler, but when we're at the top or bottom of the
         // page, the pointer may still need to move.
         move_pointer_at_page_top_and_bottom(delta);
+        last_viewport_movement_direction = delta;
     });
 
     $(window).mousewheel(function (e, delta) {

zephyr/static/js/zephyr.js

@@ -27,6 +27,9 @@ var disable_pointer_movement = false;
 // when Home is next clicked by the user
 var recenter_pointer_on_display = false;
 var suppress_scroll_pointer_update = false;
+// Includes both scroll and arrow events. Negative means scroll up,
+// positive means scroll down.
+var last_viewport_movement_direction = 1;
 
 var furthest_read = -1;
 var server_furthest_read = -1;
@@ -88,7 +91,21 @@ function recenter_view(message) {
 
     // If this logic changes, above_view_threshold andd
     // below_view_threshold must also change.
-    var selected_row_top = current_msg_list.selected_row().offset().top;
+    var selected_row = current_msg_list.selected_row();
+    var selected_row_top = selected_row.offset().top;
+
+    if ((above_view_threshold(message, true) &&
+         (last_viewport_movement_direction >= 0)) ||
+        (below_view_threshold(message) &&
+         (last_viewport_movement_direction <= 0))) {
+        // If the message you're trying to center on is already in view AND
+        // you're already trying to move in the direction of that message,
+        // don't try to recenter. This avoids disorienting jumps when the
+        // pointer has gotten itself outside the threshold (e.g. by
+        // autoscrolling).
+        return;
+    }
+
     if (above_view_threshold(message, true)) {
         // We specifically say useTop=true here, because suppose you're using
         // the arrow keys to arrow up and you've moved up to a huge message.