Fix SpriteList.insert() not rendering inserted sprite until next list sync (#2864)

* Fix SpriteList.insert() not marking index buffer as changed

SpriteList.insert() updated the CPU-side index data but never set
self._sprite_index_changed = True, unlike append() and every other
mutating method. As a result, a sprite added via insert() was present
in the list but never uploaded to the GPU index buffer, so it was not
rendered until an unrelated flag-setting operation forced a sync.

Set the flag at the end of insert() to mirror append().

Fixes #2863

* Validate texture in SpriteList.insert() to match append()

append() raises ValueError when a textureless sprite is added to an
initialized SpriteList. insert() silently accepted it. Mirror the same
guard in insert() for consistency between the two entry points.

Note: this is a validation-only change; the atlas registration itself
is already handled by _update_all() for both append() and insert().

---------

Co-authored-by: Paul Craven <paul.craven@optimizely.com>

Author: pvcraven

arcade/sprite_list/sprite_list.py

@@ -751,10 +751,15 @@ def insert(self, index: int, sprite: SpriteType) -> None:
         self._grow_index_buffer()
         self._sprite_index_data.insert(index, slot)
         self._sprite_index_data.pop()
+        self._sprite_index_changed = True
 
         if self.spatial_hash is not None:
             self.spatial_hash.add(sprite)
 
+        if self._initialized:
+            if sprite.texture is None:
+                raise ValueError("Sprite must have a texture when added to a SpriteList")
+
     def reverse(self) -> None:
         """Reverses the current list in-place"""
         # Reverse the sprites and index buffer

tests/unit/spritelist/test_spritelist.py

@@ -116,6 +116,23 @@ def test_it_can_insert_in_a_spritelist():
     assert [spritelist.sprite_slot[s] for s in spritelist] == [0, 2, 1]
     # Index buffer should refer to the slots in the same order
     assert list(spritelist._sprite_index_data[:3]) == [0, 2, 1]
+    # insert() must flag the index buffer as changed so the sprite is
+    # actually uploaded to the GPU and rendered on the next draw (#2863)
+    assert spritelist._sprite_index_changed is True
+
+
+def test_insert_requires_texture_when_initialized(ctx):
+    """insert() into an initialized list should validate the texture, like append()"""
+    spritelist = make_named_sprites(1)
+    # Force initialization (as a draw would do)
+    spritelist.draw()
+
+    sprite = arcade.SpriteSolidColor(16, 16, color=arcade.color.RED)
+    # Bypass the texture setter to simulate a textureless sprite
+    sprite._texture = None
+
+    with pytest.raises(ValueError):
+        spritelist.insert(0, sprite)
 
 
 def test_it_can_reverse_a_spritelist():