Fix datapack GUIs relying on synchronous container opening

paper-server/patches/features/0034-Restore-synchronous-container-opening.patch

@@ -0,0 +1,31 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: PaperMC <developers@papermc.io>
+Date: Sat, 18 Jul 2026 00:00:00 +0300
+Subject: [PATCH] Restore synchronous container opening
+
+Container state changes are observable from the advancement callback that
+follows a block use. Delaying an opening therefore makes the opened block
+appear closed until the next tick, breaking datapacks that use containers as
+GUIs.
+
+Only defer the close path, which can run while handling player logout or
+server shutdown. Keep opening synchronous so the block state matches vanilla
+at the time its use callback runs.
+
+diff --git a/net/minecraft/world/level/block/entity/ContainerOpenersCounter.java b/net/minecraft/world/level/block/entity/ContainerOpenersCounter.java
+index 52737172ccfa0c7c977e4f2fe1db242e0bb92d25..25b530d8880ff4fbe29a722079d1c3538f1511de 100644
+--- a/net/minecraft/world/level/block/entity/ContainerOpenersCounter.java
++++ b/net/minecraft/world/level/block/entity/ContainerOpenersCounter.java
+@@ -48,12 +48,6 @@ public abstract class ContainerOpenersCounter {
+     public void incrementOpeners(
+         final LivingEntity entity, final Level level, final BlockPos pos, final BlockState blockState, final double maxInteractionRange
+     ) {
+-        // Paper start - delay open/close callbacks
+-        if (this.delayCallbacks()) {
+-            scheduleRecheck(level, pos, blockState, 1);
+-            return;
+-        }
+-        // Paper end - delay open/close callbacks
+         int previous = this.openCount++;
+
+         // Paper start - Call BlockRedstoneEvent

paper-server/src/test/java/io/papermc/paper/world/ContainerOpenersCounterTest.java

@@ -0,0 +1,57 @@
+package io.papermc.paper.world;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import net.minecraft.core.BlockPos;
+import net.minecraft.world.entity.LivingEntity;
+import net.minecraft.world.entity.player.Player;
+import net.minecraft.world.level.Level;
+import net.minecraft.world.level.block.entity.ContainerOpenersCounter;
+import net.minecraft.world.level.block.state.BlockState;
+import org.bukkit.support.environment.Normal;
+import org.junit.jupiter.api.Test;
+import org.mockito.Mockito;
+
+@Normal
+class ContainerOpenersCounterTest {
+
+    @Test
+    void delayedCallbackContainersOpenSynchronously() {
+        final RecordingOpenersCounter counter = new RecordingOpenersCounter();
+        final LivingEntity entity = Mockito.mock(LivingEntity.class);
+        final Level level = Mockito.mock(Level.class);
+        final BlockState state = Mockito.mock(BlockState.class);
+
+        counter.incrementOpeners(entity, level, BlockPos.ZERO, state, 0.0);
+
+        assertTrue(counter.openedSynchronously);
+        assertEquals(1, counter.getOpenerCount());
+    }
+
+    private static final class RecordingOpenersCounter extends ContainerOpenersCounter {
+        private boolean openedSynchronously;
+
+        @Override
+        protected void onOpen(final Level level, final BlockPos pos, final BlockState blockState) {
+            this.openedSynchronously = true;
+        }
+
+        @Override
+        protected void onClose(final Level level, final BlockPos pos, final BlockState blockState) {
+        }
+
+        @Override
+        protected void openerCountChanged(final Level level, final BlockPos pos, final BlockState blockState, final int previous, final int current) {
+        }
+
+        @Override
+        public boolean delayCallbacks() {
+            return true;
+        }
+
+        @Override
+        public boolean isOwnContainer(final Player player) {
+            return false;
+        }
+    }
+}