Revert "in-passing removal of redundant code"

This reverts commit c81c927.

src/JBrowse/View/Layout.js

@@ -226,6 +226,7 @@ Layout.prototype.addRect = function(id, left, right, height) {
 
     var existing = {id: id, left: left, right: right,
                     top: top, height: height};
+    this.seen[id] = top;
     if (left <= this.leftBound) {
         this.leftOverlaps.push(existing);
         if (this.leftLayout) this.leftLayout.addExisting(existing);