1 files changed, 5 insertions, 2 deletions
diff --git a/src/client/util/SelectionManager.ts b/src/client/util/SelectionManager.ts
index 4612f10f4..6c386d684 100644
--- a/src/client/util/SelectionManager.ts
+++ b/src/client/util/SelectionManager.ts
@@ -55,10 +55,13 @@ export namespace SelectionManager {
         manager.SelectDoc(docView, ctrlPressed);
     }
 
+    // computed functions, such as used in IsSelected generate errors if they're called outside of a
+    // reaction context.  Specifying the context with 'outsideReaction' allows an efficiency feature
+    // to avoid unnecessary mobx invalidations when running inside a reaction.
     export function IsSelected(doc: DocumentView, outsideReaction?: boolean): boolean {
         return outsideReaction ?
-            manager.SelectedDocuments.get(doc) ? true : false :
-            computedFn(function isSelected(doc: DocumentView) {
+            manager.SelectedDocuments.get(doc) ? true : false : // get() accesses a hashtable -- setting anything in the hashtable generates a mobx invalidation for every get()
+            computedFn(function isSelected(doc: DocumentView) {  // wraapping get() in a computedFn only generates mobx() invalidations when the return value of the function for the specific get parameters has changed
                 return manager.SelectedDocuments.get(doc) ? true : false;
             })(doc);
     }