When going back to a challenge, reselect the previously-selected grou…

…p (if any), see #43

js/building/model/BuildingModel.js

@@ -66,8 +66,16 @@ define( require => {
         useDeepEquality: true
       } );
 
-      // @public {Property.<Group|null>} - We'll only show controls for this group
+      // @public {Property.<Group|null>} - We'll only show controls for this group (and track the previous value)
       this.selectedGroupProperty = new Property( null );
+      this.previouslySelectedGroupProperty = new Property( null );
+
+      // Hook up the correct values for previouslySelectedGroupProperty (no need to unlink due to same lifetime)
+      this.selectedGroupProperty.lazyLink( ( newValue, oldValue ) => {
+        if ( oldValue ) {
+          this.previouslySelectedGroupProperty.value = oldValue;
+        }
+      } );
 
       // @public {EnumerationMap.<Array.<Stack>>} - The stacks for groups
       this.groupStacksMap = new EnumerationMap( BuildingType, type => ( {

js/game/model/FractionChallenge.js

@@ -231,6 +231,16 @@ define( require => {
       return bestTarget;
     }
 
+    /**
+     * If nothing is selected, try to select the most-previously-selected group.
+     * @public
+     */
+    selectPreviouslySelectedGroup() {
+      if ( this.previouslySelectedGroupProperty.value && !this.selectedGroupProperty.value ) {
+        this.selectedGroupProperty.value = this.previouslySelectedGroupProperty.value;
+      }
+    }
+
     // REVIEW: Incomplete parameter documentation
     /**
      * Handles moving a Group into the collection area (Target).

js/game/view/BuildingGameScreenView.js

@@ -195,6 +195,9 @@ define( require => {
         lastChallengeNode = null;
         let transition;
         if ( challenge ) {
+          // See https://github.com/phetsims/fractions-common/issues/43
+          challenge.selectPreviouslySelectedGroup();
+
           const challengeNode = new FractionChallengeNode( challenge, this.layoutBounds, model.nextLevel.bind( model ), model.incorrectAttemptEmitter );
           lastChallengeNode = challengeNode;
           if ( allLevelsCompletedNode ) {