diff --git a/static/scripts/viz/trackster/tracks.js b/static/scripts/viz/trackster/tracks.js
index 902f92d09da..1507e5b7a54 100644
--- a/static/scripts/viz/trackster/tracks.js
+++ b/static/scripts/viz/trackster/tracks.js
@@ -3139,6 +3139,11 @@ extend(TiledTrack.prototype, Drawable.prototype, Track.prototype, {
         var tile_drawn = $.Deferred();
         track.tile_cache.set_elt(key, tile_drawn);
         $.when.apply($, get_tile_data()).then( function() {
+            // If deferred objects ever show up in tile data, that is likely because a
+            // Deferred-subsetting interaction failed. Specifically, a Deferred for a superset 
+            // was returned but then couldn't be used). It's not clear whether this will happen 
+            // in practice, and currently the code doesn't handle it. It could probably handle it
+            // by recursively calling draw_helper.
             var tile_data = get_tile_data(),
                 tracks_data = tile_data,
                 seq_data;
@@ -3313,10 +3318,15 @@ extend(TiledTrack.prototype, Drawable.prototype, Track.prototype, {
     },
 
     /**
-     * Returns true if data can be subsetted. Defaults to false to ensure data is fetched when needed.
+     * Returns true if data can be subsetted.
      */
     can_subset: function(data) {
-        return false;  
+        // Do not subset entries with a message or data with no detail.
+        if (data.dataset_type === 'bigwig' || data.message || data.extra_info === "no_detail")  {
+            return false;
+        }
+
+        return true;
     },
     
     /**
@@ -4116,18 +4126,6 @@ extend(FeatureTrack.prototype, Drawable.prototype, TiledTrack.prototype, {
         else {
             return true;
         }
-    },
-
-    /**
-     * Returns true if data can be subsetted.
-     */
-    can_subset: function(data) {
-        // Do not subset entries with a message or data with no detail.
-        if (data.dataset_type === 'bigwig' || data.message || data.extra_info === "no_detail")  {
-            return false;
-        }
-
-        return true;
     }
 });
 
diff --git a/static/scripts/viz/visualization.js b/static/scripts/viz/visualization.js
index 16b988745b3..c5267dec185 100644
--- a/static/scripts/viz/visualization.js
+++ b/static/scripts/viz/visualization.js
@@ -68,7 +68,7 @@ var select_datasets = function(dataset_url, add_track_async_url, filters, succes
  * Helper to determine if object is jQuery deferred.
  */
 var is_deferred = function ( d ) {
-    return ( 'isResolved' in d );
+    return ('promise' in d);
 };
 
 // --------- Models ---------
@@ -331,14 +331,6 @@ var GenomeDataManager = Cache.extend({
      * Get data from dataset.
      */
     get_data: function(region, mode, resolution, extra_params) {
-        // Debugging:
-        //console.log("get_data", low, high, mode);
-        /*
-        console.log("cache contents:")
-        for (var i = 0; i < this.key_ary.length; i++) {
-            console.log("\t", this.key_ary[i], this.obj_cache[this.key_ary[i]]);
-        }
-        */
                 
         // Look for entry and return if it's a deferred or if data available is compatible with mode.
         var entry = this.get_elt(region);
@@ -348,8 +340,7 @@ var GenomeDataManager = Cache.extend({
         }
 
         //
-        // Look in cache for data that can be used. Data can be reused if it
-        // has the requested data and is not summary tree and has details.
+        // Look in cache for data that can be used.
         // TODO: this logic could be improved if the visualization knew whether
         // the data was "index" or "data."
         //