don't increment state_id when the server modifies cursor item (#306)

## Description

The client actually doesn't acknowledge the state id update when you
modify the cursor item, so don't increment it so we stay in sync.

fixes #304

## Test Plan

Explain the steps necessary to test your changes. If you used a
playground, include the code in the details below.

Steps:
1. Run the playground from #304 (not included here for brevity)
2. open inventory (not chest)
3. pick up one of the stacks
4. see that you don't get resynced, and the apples stay in your cursor

crates/valence/src/inventory.rs

@@ -530,7 +530,9 @@ fn update_player_inventories(
         }
 
         if cursor_item.is_changed() && !inv_state.client_updated_cursor_item {
-            inv_state.state_id += 1;
+            // Contrary to what you might think, we actually don't want to increment the
+            // state ID here because the client doesn't actually acknowledge the
+            // state_id change for this packet specifically. See #304.
 
             client.write_packet(&ScreenHandlerSlotUpdateS2c {
                 window_id: -1,
@@ -1501,6 +1503,39 @@ mod test {
         Ok(())
     }
 
+    #[test]
+    fn should_not_increment_state_id_on_cursor_item_change() -> anyhow::Result<()> {
+        let mut app = App::new();
+        let (client_ent, mut client_helper) = scenario_single_client(&mut app);
+
+        let inv_state = app
+            .world
+            .get::<PlayerInventoryState>(client_ent)
+            .expect("could not find client");
+        let expected_state_id = inv_state.state_id.0;
+
+        // Process a tick to get past the "on join" logic.
+        app.update();
+        client_helper.clear_sent();
+
+        let mut cursor_item = app.world.get_mut::<CursorItem>(client_ent).unwrap();
+        cursor_item.0 = Some(ItemStack::new(ItemKind::Diamond, 2, None));
+
+        app.update();
+
+        // Make assertions
+        let inv_state = app
+            .world
+            .get::<PlayerInventoryState>(client_ent)
+            .expect("could not find client");
+        assert_eq!(
+            inv_state.state_id.0, expected_state_id,
+            "state id should not have changed"
+        );
+
+        Ok(())
+    }
+
     mod dropping_items {
         use valence_protocol::block_pos::BlockPos;
         use valence_protocol::packet::c2s::play::click_slot::{ClickMode, Slot};