Stop paint particles when no stamp lands during a held stroke

Particles kept playing at the last hit position when the cursor left the
mesh mid-stroke. New _stampLandedThisFrame flag: reset before each stamp
attempt, set only where the dab actually paints (after placement and
corner guard); no landed stamp -> StopPaintParticles immediately.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Assets/Scripts/Components/MeshPainter/MeshPainter.cs

@@ -335,6 +335,7 @@ public partial class MeshPainter : MonoBehaviour
              "(Start Color is overwritten). +Z of the instance points along the surface normal.")]
     public ParticleSystem paintParticlesPrefab;
     private ParticleSystem _paintParticles;
+    private bool _stampLandedThisFrame; // any dab actually hit the mesh this frame (gates the particles)
 
     [Header("Canvas")]
     [Tooltip("Resolution of the paint textures (square).")]
@@ -940,6 +941,7 @@ public partial class MeshPainter : MonoBehaviour
 
         if (stamp)
         {
+            _stampLandedThisFrame = false;
             BeginBackendStroke();
 
             if (!firstPress && paintInterval <= 0f)
@@ -957,6 +959,10 @@ public partial class MeshPainter : MonoBehaviour
             }
 
             EndBackendStroke();
+
+            // Held button but nothing landed (cursor off the mesh / corner guard): kill the splatter
+            // immediately instead of letting it play at the last hit until release.
+            if (!_stampLandedThisFrame) StopPaintParticles();
         }
 
         _lastScreen = screenPos;
@@ -986,6 +992,7 @@ public partial class MeshPainter : MonoBehaviour
         else UseResolvedStampMask();
 
         DrawBackendStamp(pLocal, radiusLocal, u, v, nLocal);
+        _stampLandedThisFrame = true;
         UpdatePaintParticles(pLocal, radiusLocal, nLocal);
 
         EmitStamp(pLocal, radiusLocal, u, v, nLocal);
@@ -994,8 +1001,8 @@ public partial class MeshPainter : MonoBehaviour
 
     /// <summary>Moves/scales/tints the stroke particle effect at a landed stamp and keeps it playing.
     /// Scale = brush world radius (author the prefab for radius 1, scaling mode Hierarchy), tint =
-    /// brush albedo color with a small per-particle random offset (alpha forced opaque - the brush
-    /// alpha is paint opacity, not particle fade).</summary>
+    /// brush albedo color with a small per-particle random offset; particle alpha = brush albedo
+    /// alpha (faint paint = faint splatter).</summary>
     private void UpdatePaintParticles(Vector3 pLocal, float radiusLocal, Vector3 nLocal)
     {
         if (!paintParticlesPrefab) return;
@@ -1009,11 +1016,12 @@ public partial class MeshPainter : MonoBehaviour
 
         var main = _paintParticles.main;
         Color c = brushColor;
-        c.a = 1f;
-        // Per-particle random tint offset around the albedo (darker <-> lighter).
+        // Per-particle random tint offset around the albedo (darker <-> lighter);
+        // alpha follows the brush albedo alpha (paint opacity) so faint paint splatters faintly;
+        // +0.4 bias keeps the splatter readable even at low paint opacity.
         Color dark = Color.Lerp(c, Color.black, 0.13f);
         Color light = Color.Lerp(c, Color.white, 0.09f);
-        dark.a = light.a = 1f;
+        dark.a = light.a = Mathf.Clamp01(c.a + 0.4f);
         main.startColor = new ParticleSystem.MinMaxGradient(dark, light);
 
         if (!_paintParticles.isPlaying) _paintParticles.Play();