Run stacking while performing movement in osu! composer

osu.Game.Rulesets.Osu/Beatmaps/OsuBeatmapProcessor.cs

@@ -42,22 +42,27 @@ public override void PostProcess()
         {
             base.PostProcess();
 
-            var hitObjects = Beatmap.HitObjects as List<OsuHitObject> ?? Beatmap.HitObjects.OfType<OsuHitObject>().ToList();
+            ApplyStacking(Beatmap);
+        }
+
+        internal static void ApplyStacking(IBeatmap beatmap)
+        {
+            var hitObjects = beatmap.HitObjects as List<OsuHitObject> ?? beatmap.HitObjects.OfType<OsuHitObject>().ToList();
 
             if (hitObjects.Count > 0)
             {
                 // Reset stacking
                 foreach (var h in hitObjects)
                     h.StackHeight = 0;
 
-                if (Beatmap.BeatmapInfo.BeatmapVersion >= 6)
-                    applyStacking(Beatmap.BeatmapInfo, hitObjects, 0, hitObjects.Count - 1);
+                if (beatmap.BeatmapInfo.BeatmapVersion >= 6)
+                    applyStacking(beatmap.BeatmapInfo, hitObjects, 0, hitObjects.Count - 1);
                 else
-                    applyStackingOld(Beatmap.BeatmapInfo, hitObjects);
+                    applyStackingOld(beatmap.BeatmapInfo, hitObjects);
             }
         }
 
-        private void applyStacking(BeatmapInfo beatmapInfo, List<OsuHitObject> hitObjects, int startIndex, int endIndex)
+        private static void applyStacking(BeatmapInfo beatmapInfo, List<OsuHitObject> hitObjects, int startIndex, int endIndex)
         {
             ArgumentOutOfRangeException.ThrowIfGreaterThan(startIndex, endIndex);
             ArgumentOutOfRangeException.ThrowIfNegative(startIndex);
@@ -209,7 +214,7 @@ private void applyStacking(BeatmapInfo beatmapInfo, List<OsuHitObject> hitObject
             }
         }
 
-        private void applyStackingOld(BeatmapInfo beatmapInfo, List<OsuHitObject> hitObjects)
+        private static void applyStackingOld(BeatmapInfo beatmapInfo, List<OsuHitObject> hitObjects)
         {
             for (int i = 0; i < hitObjects.Count; i++)
             {

osu.Game.Rulesets.Osu/Edit/OsuHitObjectComposer.cs

@@ -295,6 +295,12 @@ private bool snapToVisibleBlueprints(Vector2 screenSpacePosition, out SnapResult
 
                 if (Vector2.Distance(closestSnapPosition, screenSpacePosition) < snapRadius)
                 {
+                    // if the snap target is a stacked object, snap to its unstacked position rather than its stacked position.
+                    // this is intended to make working with stacks easier (because thanks to this, you can drag an object to any
+                    // of the items on the stack to add an object to it, rather than having to drag to the position of the *first* object on it at all times).
+                    if (b.Item is OsuHitObject osuObject && osuObject.StackOffset != Vector2.Zero)
+                        closestSnapPosition = b.ToScreenSpace(b.ToLocalSpace(closestSnapPosition) - osuObject.StackOffset);
+
                     // only return distance portion, since time is not really valid
                     snapResult = new SnapResult(closestSnapPosition, null, playfield);
                     return true;

osu.Game.Rulesets.Osu/Edit/OsuSelectionHandler.cs

@@ -13,6 +13,7 @@
 using osu.Game.Rulesets.Edit;
 using osu.Game.Rulesets.Objects;
 using osu.Game.Rulesets.Objects.Types;
+using osu.Game.Rulesets.Osu.Beatmaps;
 using osu.Game.Rulesets.Osu.Objects;
 using osu.Game.Rulesets.Osu.UI;
 using osu.Game.Screens.Edit.Compose.Components;
@@ -50,12 +51,33 @@ public override bool HandleMovement(MoveSelectionEvent<HitObject> moveEvent)
         {
             var hitObjects = selectedMovableObjects;
 
+            var localDelta = this.ScreenSpaceDeltaToParentSpace(moveEvent.ScreenSpaceDelta);
+
+            // this conditional is a rather ugly special case for stacks.
+            // as it turns out, adding the `EditorBeatmap.Update()` call at the end of this would cause stacked objects to jitter when moved around
+            // (they would stack and then unstack every frame).
+            // the reason for that is that the selection handling abstractions are not aware of the distinction between "displayed" and "actual" position
+            // which is unique to osu! due to stacking being applied as a post-processing step.
+            // therefore, the following loop would occur:
+            // - on frame 1 the blueprint is snapped to the stack's baseline position. `EditorBeatmap.Update()` applies stacking successfully,
+            //   the blueprint moves up the stack from its original drag position.
+            // - on frame 2 the blueprint's position is now the *stacked* position, which is interpreted higher up as *manually performing an unstack*
+            //   to the blueprint's unstacked position (as the machinery higher up only cares about differences in screen space position).
+            if (hitObjects.Any(h => Precision.AlmostEquals(localDelta, -h.StackOffset)))
+                return true;
+
             // this will potentially move the selection out of bounds...
             foreach (var h in hitObjects)
-                h.Position += this.ScreenSpaceDeltaToParentSpace(moveEvent.ScreenSpaceDelta);
+                h.Position += localDelta;
 
             // but this will be corrected.
             moveSelectionInBounds();
+
+            // manually update stacking.
+            // this intentionally bypasses the editor `UpdateState()` / beatmap processor flow for performance reasons,
+            // as the entire flow is too expensive to run on every movement.
+            Scheduler.AddOnce(OsuBeatmapProcessor.ApplyStacking, EditorBeatmap);
+
             return true;
         }