--- a/net/minecraft/server/level/ChunkHolder.java
+++ b/net/minecraft/server/level/ChunkHolder.java
@@ -37,6 +36,10 @@
 import net.minecraft.world.level.chunk.ProtoChunk;
 import net.minecraft.world.level.lighting.LevelLightEngine;
 
+// CraftBukkit start
+import net.minecraft.server.MinecraftServer;
+// CraftBukkit end
+
 public class ChunkHolder {
     public static final Either<ChunkAccess, ChunkHolder.ChunkLoadingFailure> UNLOADED_CHUNK = Either.right(ChunkHolder.ChunkLoadingFailure.UNLOADED);
     public static final CompletableFuture<Either<ChunkAccess, ChunkHolder.ChunkLoadingFailure>> UNLOADED_CHUNK_FUTURE = CompletableFuture.completedFuture(
@@ -93,14 +94,17 @@
         this.changedBlocksPerSection = new ShortSet[levelHeightAccessor.getSectionsCount()];
     }
 
-    public CompletableFuture<Either<ChunkAccess, ChunkHolder.ChunkLoadingFailure>> getFutureIfPresentUnchecked(ChunkStatus chunkStatus) {
-        CompletableFuture<Either<ChunkAccess, ChunkHolder.ChunkLoadingFailure>> completableFuture = this.futures.get(chunkStatus.getIndex());
-        return completableFuture == null ? UNLOADED_CHUNK_FUTURE : completableFuture;
+    // CraftBukkit start
+    public LevelChunk getFullChunkNow() {
+        // Note: We use the oldTicketLevel for isLoaded checks.
+        if (!ChunkLevel.fullStatus(this.oldTicketLevel).isOrAfter(FullChunkStatus.FULL)) return null;
+        return this.getFullChunkNowUnchecked();
     }
 
     public CompletableFuture<Either<ChunkAccess, ChunkHolder.ChunkLoadingFailure>> getFutureIfPresent(ChunkStatus chunkStatus) {
         return ChunkLevel.generationStatus(this.ticketLevel).isOrAfter(chunkStatus) ? this.getFutureIfPresentUnchecked(chunkStatus) : UNLOADED_CHUNK_FUTURE;
     }
+    // CraftBukkit end
 
     public CompletableFuture<Either<LevelChunk, ChunkHolder.ChunkLoadingFailure>> getTickingChunkFuture() {
         return this.tickingChunkFuture;
@@ -171,10 +192,13 @@
     }
 
     public void blockChanged(BlockPos pos) {
-        LevelChunk tickingChunk = this.getTickingChunk();
-        if (tickingChunk != null) {
-            int sectionIndex = this.levelHeightAccessor.getSectionIndex(pos.getY());
-            if (this.changedBlocksPerSection[sectionIndex] == null) {
+        LevelChunk chunk = this.getTickingChunk();
+
+        if (chunk != null) {
+            int i = this.levelHeightAccessor.getSectionIndex(pos.getY());
+
+            if (i < 0 || i >= this.changedBlocksPerSection.length) return; // CraftBukkit - SPIGOT-6086, SPIGOT-6296
+            if (this.changedBlocksPerSection[i] == null) {
                 this.hasChangedSections = true;
                 this.changedBlocksPerSection[sectionIndex] = new ShortOpenHashSet();
             }
@@ -238,14 +272,16 @@
                                 this.broadcast(players, new ClientboundBlockUpdatePacket(blockPos, blockState));
                                 this.broadcastBlockEntityIfNeeded(players, level, blockPos, blockState);
                             } else {
-                                LevelChunkSection section = chunk.getSection(i);
-                                ClientboundSectionBlocksUpdatePacket clientboundSectionBlocksUpdatePacket = new ClientboundSectionBlocksUpdatePacket(
-                                    sectionPos, set, section
-                                );
-                                this.broadcast(players, clientboundSectionBlocksUpdatePacket);
-                                clientboundSectionBlocksUpdatePacket.runUpdates(
-                                    (blockPos1, blockState1) -> this.broadcastBlockEntityIfNeeded(players, level, blockPos1, blockState1)
-                                );
+                                LevelChunkSection chunksection = chunk.getSection(i);
+                                ClientboundSectionBlocksUpdatePacket packetplayoutmultiblockchange = new ClientboundSectionBlocksUpdatePacket(sectionposition, shortset, chunksection);
+
+                                this.broadcast(list, packetplayoutmultiblockchange);
+                                // CraftBukkit start
+                                List finalList = list;
+                                packetplayoutmultiblockchange.runUpdates((blockposition1, iblockdata1) -> {
+                                    this.broadcastBlockEntityIfNeeded(finalList, world, blockposition1, iblockdata1);
+                                    // CraftBukkit end
+                                });
                             }
                         }
                     }
@@ -368,15 +427,39 @@
     }
 
     protected void updateFutures(ChunkMap chunkMap, Executor executor) {
-        ChunkStatus chunkStatus = ChunkLevel.generationStatus(this.oldTicketLevel);
-        ChunkStatus chunkStatus1 = ChunkLevel.generationStatus(this.ticketLevel);
-        boolean isLoaded = ChunkLevel.isLoaded(this.oldTicketLevel);
-        boolean isLoaded1 = ChunkLevel.isLoaded(this.ticketLevel);
-        FullChunkStatus fullChunkStatus = ChunkLevel.fullStatus(this.oldTicketLevel);
-        FullChunkStatus fullChunkStatus1 = ChunkLevel.fullStatus(this.ticketLevel);
-        if (isLoaded) {
-            Either<ChunkAccess, ChunkHolder.ChunkLoadingFailure> either = Either.right(new ChunkHolder.ChunkLoadingFailure() {
-                @Override
+        ChunkStatus chunkstatus = ChunkLevel.generationStatus(this.oldTicketLevel);
+        ChunkStatus chunkstatus1 = ChunkLevel.generationStatus(this.ticketLevel);
+        boolean flag = ChunkLevel.isLoaded(this.oldTicketLevel);
+        boolean flag1 = ChunkLevel.isLoaded(this.ticketLevel);
+        FullChunkStatus fullchunkstatus = ChunkLevel.fullStatus(this.oldTicketLevel);
+        FullChunkStatus fullchunkstatus1 = ChunkLevel.fullStatus(this.ticketLevel);
+        // CraftBukkit start
+        // ChunkUnloadEvent: Called before the chunk is unloaded: isChunkLoaded is still true and chunk can still be modified by plugins.
+        if (fullchunkstatus.isOrAfter(FullChunkStatus.FULL) && !fullchunkstatus1.isOrAfter(FullChunkStatus.FULL)) {
+            this.getFutureIfPresentUnchecked(ChunkStatus.FULL).thenAccept((either) -> {
+                LevelChunk chunk = (LevelChunk)either.left().orElse(null);
+                if (chunk != null) {
+                    chunkMap.callbackExecutor.execute(() -> {
+                        // Minecraft will apply the chunks tick lists to the world once the chunk got loaded, and then store the tick
+                        // lists again inside the chunk once the chunk becomes inaccessible and set the chunk's needsSaving flag.
+                        // These actions may however happen deferred, so we manually set the needsSaving flag already here.
+                        chunk.setUnsaved(true);
+                        chunk.unloadCallback();
+                    });
+                }
+            }).exceptionally((throwable) -> {
+                // ensure exceptions are printed, by default this is not the case
+                MinecraftServer.LOGGER.error("Failed to schedule unload callback for chunk " + ChunkHolder.this.pos, throwable);
+                return null;
+            });
+
+            // Run callback right away if the future was already done
+            chunkMap.callbackExecutor.run();
+        }
+        // CraftBukkit end
+
+        if (flag) {
+            Either<ChunkAccess, ChunkHolder.Failure> either = Either.right(new ChunkHolder.Failure() {
                 public String toString() {
                     return "Unloaded ticket level " + ChunkHolder.this.pos;
                 }
@@ -440,6 +527,26 @@
 
         this.onLevelChange.onLevelChange(this.pos, this::getQueueLevel, this.ticketLevel, this::setQueueLevel);
         this.oldTicketLevel = this.ticketLevel;
+        // CraftBukkit start
+        // ChunkLoadEvent: Called after the chunk is loaded: isChunkLoaded returns true and chunk is ready to be modified by plugins.
+        if (!fullchunkstatus.isOrAfter(FullChunkStatus.FULL) && fullchunkstatus1.isOrAfter(FullChunkStatus.FULL)) {
+            this.getFutureIfPresentUnchecked(ChunkStatus.FULL).thenAccept((either) -> {
+                LevelChunk chunk = (LevelChunk)either.left().orElse(null);
+                if (chunk != null) {
+                    chunkMap.callbackExecutor.execute(() -> {
+                        chunk.loadCallback();
+                    });
+                }
+            }).exceptionally((throwable) -> {
+                // ensure exceptions are printed, by default this is not the case
+                MinecraftServer.LOGGER.error("Failed to schedule load callback for chunk " + ChunkHolder.this.pos, throwable);
+                return null;
+            });
+
+            // Run callback right away if the future was already done
+            chunkMap.callbackExecutor.run();
+        }
+        // CraftBukkit end
     }
 
     public boolean wasAccessibleSinceLastSave() {