Add comment about last button invocation

osu.Game/Overlays/Dialog/PopupDialog.cs

@@ -241,7 +241,10 @@ protected override void PopIn()
 
         protected override void PopOut()
         {
-            if (!actionInvoked) buttonsContainer.Last().TriggerOnClick();
+            if (!actionInvoked)
+                // In the case a user did not choose an action before a hide was triggered, press the last button.
+                // This is presumed to always be a sane default "cancel" action.
+                buttonsContainer.Last().TriggerOnClick();
 
             base.PopOut();
             content.FadeOut(EXIT_DURATION, Easing.InSine);