Huge speedup in get_next_visible/get_prev_visible.

nextAll/prevAll walks the entire DOM, basically.
This code only walks the DOM until we find a new .message_row.

This speeds up the average time of a call to this function from about
6.38ms to 0.678ms, in my benchmarking.

Admittedly, the whole outer loop here could still use some
optimization, if we want to; do we really need to call this 1000
times?

(imported from commit 852e2f660a16f8cfd7be35d3271aedb1ac481663)

zephyr/static/js/dom_access.js

@@ -1,13 +1,18 @@
+// We need to andSelf() because more often than not, the next item
+// *is* a .message_row, so nextUntil returns an empty set (except for
+// when it's in a bookend).
+// (This could probably be further optimized by handling that case
+// explicitly first, since it's also the common case.)
 function get_next_visible(message_row) {
     if (message_row === undefined)
         return [];
-    return message_row.nextAll('.message_row').filter(':first');
+    return message_row.nextUntil('.message_row').andSelf().next('.message_row');
 }
 
 function get_prev_visible(message_row) {
     if (message_row === undefined)
         return [];
-    return message_row.prevAll('.message_row').filter(':first');
+    return message_row.prevUntil('.message_row').andSelf().prev('.message_row');
 }
 
 function get_first_visible() {