Ender Dragon spawn egg: vanilla random order and the dragon's own random

EntityType.create draws the wrapped spawn yaw and Mob.finalizeSpawn the
follow-range triangle and the left-handed roll from the level random;
the dragon seeds its own random (Entity.random), not from the level one.
test_spawn_egg_fluid_parity gains the dragon egg used on a block.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/horizon-client/src/game/dragon.rs

@@ -4177,18 +4177,25 @@ impl Game<'_> {
         true
     }
 
-    /// Create an Ender Dragon for a scripted spawn such as a spawn egg or a
-    /// dispenser.  Unlike the End fight's dragon, this one is not assigned to
-    /// the fight UUID and can therefore be summoned in any dimension.
+    /// `EntityType.create` for an Ender Dragon from a spawn egg, by hand or from a dispenser
+    /// (`SpawnEggItem.spawnMob`, `SpawnEggItemBehavior`). Unlike the End fight's dragon, this
+    /// one is not the fight's (`EnderDragon.aiStep` adopts it only when its UUID is the fight's),
+    /// so it can be spawned in any dimension. Horizon keeps one dragon per level, as `/summon`
+    /// and loading do: with one already there, nothing spawns and the egg stays.
     pub(super) fn spawn_dragon_spawned(&mut self, at: [f64; 3]) -> Option<u32> {
-        // Horizon currently keeps one simulated dragon per level, matching
-        // the existing `/summon`/save representation.
         if self.dragon.is_some() {
             return None;
         }
         let position = Vec3::new(at[0], at[1], at[2]);
-        let yaw = self.world.random.next_float() * 360.0;
-        let seed = self.world.random.next_long();
+        // EntityType.create: `snapTo(.., Mth.wrapDegrees(level.getRandom().nextFloat() * 360))`.
+        let yaw = horizon_client::camera::wrap_degrees(self.world.random.next_float() * 360.0);
+        // Mob.finalizeSpawn (EnderDragon keeps it): the follow range's
+        // `random.triangle(0, 0.11485)`, then `setLeftHanded(nextFloat() < 0.05)`, from the
+        // level's random. Neither changes how the dragon flies.
+        let _ = self.world.random.triangle(0.0, 0.114_850_000_000_000_01);
+        let _ = self.world.random.next_float();
+        // The dragon's own random (`Entity.random`), not the level's.
+        let seed = super::dispenser::entity_random().next_long();
         let mut dragon = Dragon::new(position, yaw, seed);
         dragon.id = self.next_entity + 1;
         self.next_entity += 9;

tools/test_spawn_egg_fluid_parity.py

@@ -4,7 +4,8 @@
 Fluid cases send UseItem into a deep pool; a UseItemOn control has a mob
 under the last look direction. Compare spawn, egg count and used stat. Covers
 lava/water sources, flowing lava, a solid obstruction, waterlogged blocks,
-creative, adventure (with/without can_place_on) and peaceful monster eggs.
+creative, adventure (with/without can_place_on) and peaceful monster eggs, and the
+Ender Dragon's egg on a block (Horizon simulates the dragon apart from other mobs).
 """
 import json
 try:
@@ -24,6 +25,7 @@ CASES = [
     ('adventure-allowed', 'lava', 'strider', 'adventure',
      '[minecraft:can_place_on={blocks:"minecraft:lava"}]', True),
     ('peaceful', 'lava', 'zombie', 'survival', '', False),
+    ('block-use-dragon', 'stone', 'ender_dragon', 'survival', '', True),
 ]
 SCRIPT = [("setup", ["chat on", "cmd Alice gamerule spawn_mobs false"])]
 previous_mode = "creative"
@@ -37,15 +39,15 @@ for name, block, mob, mode, component, spawn in CASES:
         'cmd Alice fill -2 74 -2 6 80 6 minecraft:stone',
         'cmd Alice fill -2 81 -2 6 85 6 minecraft:air',
         f'cmd Alice fill 0 75 0 4 80 4 minecraft:{block}',
-        f'cmd Alice tp @s 2.5 81 -0.5 0 {0 if name == "block-use" else 50}',
-        *(['cmd Alice summon minecraft:strider 2.5 81 1.5 {NoAI:1b}'] if name == 'block-use' else []),
+        f'cmd Alice tp @s 2.5 81 -0.5 0 {0 if name.startswith("block-use") else 50}',
+        *(['cmd Alice summon minecraft:strider 2.5 81 1.5 {NoAI:1b}'] if name.startswith('block-use') else []),
         f'cmd Alice give @s {egg}{component} 8',
         'cmd Alice scoreboard objectives remove eggs',
         f'cmd Alice scoreboard objectives add eggs minecraft.used:minecraft.{mob}_spawn_egg',
         *([f'cmd Alice gamemode {mode}'] if mode != 'creative' else []),
     ]
     previous_mode = mode
-    SCRIPT += [(name + '-setup', setup), (name + '-use', ['use-on Alice 4 80 -1' if name == 'block-use' else 'use Alice 50']),
+    SCRIPT += [(name + '-setup', setup), (name + '-use', ['use-on Alice 4 80 -1' if name.startswith('block-use') else 'use Alice 50']),
                (name + '-check', [
                    f'cmd Alice execute {"if" if spawn else "unless"} entity @e[type=minecraft:{mob}]',
                    f'cmd Alice clear @s {egg} 0',