Ensure phantom trail occurs only in motion (#37)

developer_documentation.md

@@ -8,6 +8,7 @@
    * [Offhand Animation](#Offhand-animation)
    * [Particles](#Particles)
       * [Sweep Attack](#Sweep-attack)
+   * [Phantoms](#Phantoms)
    * [Player skin parts](#Player-skin-parts)
    * [Shulkers](#Shulkers)
    * [Spectral arrow entities](#Spectral-arrow-entities)
@@ -126,6 +127,41 @@ convert -append extracted/assets/minecraft/textures/particle/sweep_*.png -define
 
 The `-append` flag is used to join the input images which match the defined globular expression (`.../sweep_*.png`). The image format is defined for safety as by default Imagemagick will attempt to change the color mode of the image to grayscale, which Minecraft will not interpret correctly. The image is then placed in the pack at the defined path.
 
+### Phantoms
+
+Because Geyser does not utilize the 1.17.40 subchunk system, phantom trail particles are attached to the player entity and fail to despawn when a phantom spawner is present. This seems to be related to how the client perceive the phantom "entity" attached to the spawner. This can be mitigated by adding an additional state to the phantom's animation controller, which ensures its trail particle is only played when the phantom is in motion.
+
+```json
+{
+  "controller.animation.phantom.base_pose": {
+    "initial_state": "default",
+    "states": {
+        "default": {
+            "transitions": [
+                {
+                    "moving": "q.is_moving"
+                }
+            ],
+            "animations": [ "phantom_base_pose" ]
+        },
+        "moving": {
+            "animations": [ "phantom_base_pose" ],
+            "particle_effects": [
+                {
+                    "effect": "wing_dust",
+                    "locator": "left_wing"
+                },
+                {
+                    "effect": "wing_dust",
+                    "locator": "right_wing"
+                }
+            ]
+        }
+    }
+  }
+}
+```
+
 ### Player skin parts
 
 On Java Edition, you are able to toggle your cape and second skin layers. Bedrock Edition does not share this property. We're able to solve this by applying Java's player skin parts mask to the `q.mark_variant` query and checking the visibility with this formula in the player render controller and cape render controller: