Scroll more like barnowl again. (but closer to the edges)

We are moving back to a barnowl-ish scrolling algorithm for
the arrow keys, where when you have a message selected toward
the bottom of the screen, hitting down arrow and up arrow
effectively puts the originally selected message at the center
of the screen.  In order to avoid unnecessary scrolls, we
are making it so that you can move the pointer closer to the
edges.

(imported from commit c08233d6d2034a04469b8f424b39d94a230cafe0)

zephyr/static/js/zephyr.js

@@ -83,21 +83,18 @@ function within_viewport(row_offset, row_height) {
 
 function recenter_view(message, from_scroll) {
     // Barnowl-style recentering: if the pointer is too high, move it to
-    // the 4/5 marks. If the pointer is too low, move it to the 1/5 mark.
+    // the 1/2 marks. If the pointer is too low, move it to the 1/7 mark.
     // See keep_pointer_in_view() for related logic to keep the pointer onscreen.
 
     var viewport_info = ui.message_viewport_info();
-    var top_threshold =
+    var top_threshold = viewport_info.visible_top;
-        viewport_info.visible_top +
-        (1/5 * viewport_info.visible_height);
 
-    var bottom_threshold =
+    var bottom_threshold = viewport_info.visible_top + viewport_info.visible_height;
-        viewport_info.visible_top +
-        (4/5 * viewport_info.visible_height);
 
     var message_top = message.offset().top;
+    var message_bottom = message_top + message.outerHeight(true);
     var is_above = message_top < top_threshold;
-    var is_below = message_top > bottom_threshold;
+    var is_below = message_bottom > bottom_threshold;
 
     if (from_scroll) {
         // If the message you're trying to center on is already in view AND
@@ -114,9 +111,9 @@ function recenter_view(message, from_scroll) {
     }
 
     if (is_above) {
-        viewport.set_message_position(message_top, viewport_info, 4/5);
+        viewport.set_message_position(message_top, viewport_info, 1/2);
     } else if (is_below) {
-        viewport.set_message_position(message_top, viewport_info, 1/5);
+        viewport.set_message_position(message_top, viewport_info, 1/7);
     }
 }
 
@@ -1144,6 +1141,11 @@ setInterval(function () {
 
 function keep_pointer_in_view() {
     // See recenter_view() for related logic to keep the pointer onscreen.
+    // This function mostly comes into place for mouse scrollers, and it
+    // keeps the pointer in view.  For people who purely scroll with the
+    // mouse, the pointer is kind of meaningless to them, but keyboard
+    // users will occasionally do big mouse scrolls, so this gives them
+    // a pointer reasonably close to the middle of the screen.
     var candidate;
     var next_row = current_msg_list.selected_row();
 
@@ -1151,8 +1153,8 @@ function keep_pointer_in_view() {
         return;
 
     var info = ui.message_viewport_info();
-    var top_threshold = info.visible_top + (1/5 * info.visible_height);
+    var top_threshold = info.visible_top + (1/10 * info.visible_height);
-    var bottom_threshold = info.visible_top + (4/5 * info.visible_height);
+    var bottom_threshold = info.visible_top + (9/10 * info.visible_height);
 
     function above_view_threshold() {
         var bottom_offset = next_row.offset().top + next_row.outerHeight(true);