Block input to objects lying under already-hit hitcircles when classic note lock is active

osu.Game.Rulesets.Osu.Tests/TestSceneLegacyHitPolicy.cs

@@ -549,6 +549,8 @@ namespace osu.Game.Rulesets.Osu.Tests
             addJudgementOffsetAssert("first slider head", () => ((Slider)hitObjects[0]).HeadCircle, 0);
             addJudgementAssert(hitObjects[1], HitResult.Miss);
             // the slider head of the first slider prevents the second slider's head from being hit, so the judgement offset should be very late.
+            // this is not strictly done by the hit policy implementation itself (see `OsuModClassic.blockInputToUnderlyingObjects()`),
+            // but we're testing this here anyways to just keep everything related to input handling and note lock in one place.
             addJudgementOffsetAssert("second slider head", () => ((Slider)hitObjects[1]).HeadCircle, referenceHitWindows.WindowFor(HitResult.Meh));
             addClickActionAssert(0, ClickAction.Hit);
         }

osu.Game.Rulesets.Osu/Mods/OsuModClassic.cs

@@ -74,6 +74,10 @@ namespace osu.Game.Rulesets.Osu.Mods
                     head.TrackFollowCircle = !NoSliderHeadMovement.Value;
                     if (FadeHitCircleEarly.Value && !usingHiddenFading)
                         applyEarlyFading(head);
+
+                    if (ClassicNoteLock.Value)
+                        blockInputToUnderlyingObjects(head);
+
                     break;
 
                 case DrawableSliderTail tail:
@@ -83,10 +87,29 @@ namespace osu.Game.Rulesets.Osu.Mods
                 case DrawableHitCircle circle:
                     if (FadeHitCircleEarly.Value && !usingHiddenFading)
                         applyEarlyFading(circle);
+
+                    if (ClassicNoteLock.Value)
+                        blockInputToUnderlyingObjects(circle);
+
                     break;
             }
         }
 
+        /// <summary>
+        /// On stable, hitcircles that have already been hit block input from reaching objects that may be underneath them.
+        /// The purpose of this method is to restore that behaviour.
+        /// In order to avoid introducing yet another confusing config option, this behaviour is roped into the general notion of "note lock".
+        /// </summary>
+        private static void blockInputToUnderlyingObjects(DrawableHitCircle circle)
+        {
+            var oldHitAction = circle.HitArea.Hit;
+            circle.HitArea.Hit = () =>
+            {
+                oldHitAction?.Invoke();
+                return true;
+            };
+        }
+
         private void applyEarlyFading(DrawableHitCircle circle)
         {
             circle.ApplyCustomUpdateState += (dho, state) =>

osu.Game.Rulesets.Osu/Objects/Drawables/DrawableHitCircle.cs

@@ -261,7 +261,7 @@ namespace osu.Game.Rulesets.Osu.Objects.Drawables
                     case OsuAction.RightButton:
                         if (IsHovered && (Hit?.Invoke() ?? false))
                         {
-                            HitAction = e.Action;
+                            HitAction ??= e.Action;
                             return true;
                         }