Fixes #1532. Also fixes the Sable side of Creators-of-Create/Create#10255.

.gitignore

@@ -18,6 +18,9 @@
 
 # IntelliJ
 out/
+
+# Eclipse / VS Code Java language server build output
+bin/
 # mpeltonen/sbt-idea plugin
 .idea_modules/
 

common/src/main/java/dev/ryanhcode/sable/api/SubLevelAssemblyHelper.java

@@ -402,6 +402,11 @@ public static void moveBlocks(final ServerLevel level, final AssemblyTransform t
                         container.setLootTable(null);
                     }
                     Clearable.tryClear(blockEntity);
+                    if (blockEntity != null && !(blockEntity instanceof Clearable)) {
+                        // The destination copy already owns the contents; leaving them in the source block entity
+                        // would make the destruction below drop (or void) a duplicate via onRemove(...).
+                        level.removeBlockEntity(block);
+                    }
                 }
 
                 final LevelChunk chunk = resultingAccelerator.getChunk(SectionPos.blockToSectionCoord(newPos.getX()), SectionPos.blockToSectionCoord(newPos.getZ()));

common/src/main/java/dev/ryanhcode/sable/api/sublevel/SubLevelContainer.java

@@ -154,7 +154,7 @@ public void processSubLevelRemovals() {
         for (final SubLevel subLevel : this.allSubLevels) {
             if (subLevel instanceof final ServerSubLevel serverSubLevel) {
                 if (!serverSubLevel.isRemoved() && serverSubLevel.getMassTracker().isInvalid()) {
-                    serverSubLevel.getPlot().destroyAllBlocks();
+                    serverSubLevel.getPlot().destroyAllBlocks(false);
                     serverSubLevel.markRemoved();
                 }
             }

common/src/main/java/dev/ryanhcode/sable/sublevel/plot/ServerLevelPlot.java

@@ -291,8 +291,12 @@ public void kickAllEntities() {
 
     /**
      * Destroys all blocks within the plot
+     *
+     * @param dropItems whether the destroyed blocks should drop their items. Mass invalidation must pass
+     *                  {@code false}: at that point the plot only ever contains air and zero-mass blocks, which the
+     *                  disassembly sweep has already moved back to the world.
      */
-    public void destroyAllBlocks() {
+    public void destroyAllBlocks(boolean dropItems) {
         if (this.localBounds == null || this.localBounds == BoundingBox3i.EMPTY) {
             return;
         }
@@ -305,7 +309,20 @@ public void destroyAllBlocks() {
                 for (int z = bounds.minZ(); z <= bounds.maxZ(); z++) {
                     final BlockPos pos = new BlockPos(x, y, z);
 
-                    level.destroyBlock(pos, true);
+                    if (dropItems) {
+                        level.destroyBlock(pos, true);
+                    } else {
+                        final BlockState state = level.getBlockState(pos);
+                        if (state.isAir()) {
+                            continue;
+                        }
+                        // Detach the block entity first so onRemove(...) cannot spill container contents. Neighbor
+                        // updates are suppressed as well: nothing needs to react to a wholesale plot deletion, and
+                        // updates can pop foreign blocks living in adjacent plot grid cells (dropping them).
+                        level.removeBlockEntity(pos);
+                        level.setBlock(pos, state.getFluidState().createLegacyBlock(),
+                                Block.UPDATE_CLIENTS | Block.UPDATE_MOVE_BY_PISTON);
+                    }
                 }
             }
         }

common/src/main/java/dev/ryanhcode/sable/sublevel/plot/heat/SubLevelHeatMapManager.java

@@ -243,7 +243,7 @@ private void split() {
 
             // Protect against split sub-levels that have zero mass.
             if (subLevel.getSelfMassTracker().getCenterOfMass() == null || subLevel.getSelfMassTracker().getMass() <= 0.0) {
-                subLevel.getPlot().destroyAllBlocks();
+                subLevel.getPlot().destroyAllBlocks(true);
 
                 final SubLevelContainer container = Objects.requireNonNull(SubLevelContainer.getContainer(level));
                 container.removeSubLevel(subLevel, SubLevelRemovalReason.REMOVED);

common/src/main/java/dev/ryanhcode/sable/sublevel/system/SubLevelPhysicsSystem.java

@@ -531,7 +531,7 @@ public void updateMassDataFromBlockChange(final SubLevel subLevel, final BlockPo
                 if (oldMass != 0.0) massTracker.addBlockMass(level, oldState, globalBlockPos, -oldMass, oldInertia);
 
                 if (!subLevel.isRemoved() && massTracker.isInvalid()) {
-                    serverSubLevel.getPlot().destroyAllBlocks();
+                    serverSubLevel.getPlot().destroyAllBlocks(false);
                     serverSubLevel.markRemoved();
                     return;
                 }

neoforge/src/main/java/dev/ryanhcode/sable/neoforge/gametest/AssemblyTest.java

@@ -24,11 +24,14 @@
 import net.minecraft.resources.ResourceLocation;
 import net.minecraft.server.level.ServerLevel;
 import net.minecraft.world.entity.Entity;
+import net.minecraft.world.entity.EntityType;
+import net.minecraft.world.entity.item.ItemEntity;
 import net.minecraft.world.item.ItemStack;
 import net.minecraft.world.item.Items;
 import net.minecraft.world.level.Level;
 import net.minecraft.world.level.block.Block;
 import net.minecraft.world.level.block.Blocks;
+import net.minecraft.world.level.block.WallTorchBlock;
 import net.minecraft.world.level.block.state.BlockState;
 import net.minecraft.world.level.block.state.properties.BlockStateProperties;
 import net.minecraft.world.level.block.state.properties.Property;
@@ -118,6 +121,81 @@ public static void testBrittleBreaking(final GameTestHelper helper) {
         });
     }
 
+    /**
+     * Regression test: demolishing an invalidated (massless) plot must not drop anything — the weightless blocks
+     * left in the plot are duplicates of blocks the disassembly sweep has already moved back to the world.
+     */
+    @GameTest(template = "brittlebreak")
+    public static void testMasslessInvalidationDoesNotDrop(final GameTestHelper helper) {
+        final ServerLevel level = helper.getLevel();
+        final ServerSubLevelContainer plotContainer = SubLevelContainer.getContainer(level);
+        if (plotContainer == null) {
+            throw new IllegalStateException("Plot container not found in level");
+        }
+
+        final BlockPos min = helper.absolutePos(new BlockPos(0, 1, 0));
+        final BlockPos max = helper.absolutePos(new BlockPos(2, 3, 2));
+        final BoundingBox3i bounds = new BoundingBox3i(
+                min.getX(), min.getY(), min.getZ(),
+                max.getX(), max.getY(), max.getZ()
+        );
+
+        // A single stone with a zero-mass wall torch on each of its four sides (covering every facing).
+        final BlockPos stonePos = new BlockPos(1, 1, 1);
+
+        // Reset the template area — the floor layer included, or stray template decorations can drop during the
+        // run — so the test does not depend on template contents. Shape updates are suppressed (flag 16): clearing
+        // with normal flags would pop the template's adjacent redstone components and drop their items.
+        for (final BlockPos pos : BlockPos.betweenClosed(new BlockPos(0, 0, 0), new BlockPos(2, 3, 2))) {
+            level.setBlock(helper.absolutePos(pos), Blocks.AIR.defaultBlockState(),
+                    Block.UPDATE_CLIENTS | Block.UPDATE_KNOWN_SHAPE);
+        }
+        helper.setBlock(stonePos, Blocks.STONE.defaultBlockState());
+        for (final Direction direction : Direction.Plane.HORIZONTAL) {
+            helper.setBlock(stonePos.relative(direction),
+                    Blocks.WALL_TORCH.defaultBlockState().setValue(WallTorchBlock.FACING, direction));
+        }
+
+        final ServerSubLevel subLevel = SubLevelAssemblyHelper.assembleBlocks(level, min, BlockPos.betweenClosed(min, max), bounds);
+
+        helper.runAtTickTime(10, () -> {
+            final Level plot = subLevel.getLevel();
+            final BoundingBox3ic plotBounds = subLevel.getPlot().getBoundingBox();
+
+            // Remove the only mass-bearing block. Shape updates are suppressed (flag 16), matching silent
+            // disassembly sweeps, so the zero-mass torches are not popped first. This invalidates the mass
+            // tracker, and the physics system demolishes the plot through the real invalidation path.
+            for (final BlockPos pos : BlockPos.betweenClosed(plotBounds.minX(), plotBounds.minY(), plotBounds.minZ(),
+                    plotBounds.maxX(), plotBounds.maxY(), plotBounds.maxZ())) {
+                if (plot.getBlockState(pos).isAir() || plot.getBlockState(pos).getBlock() == Blocks.WALL_TORCH) {
+                    continue;
+                }
+                plot.setBlock(pos, Blocks.AIR.defaultBlockState(), Block.UPDATE_CLIENTS | Block.UPDATE_KNOWN_SHAPE);
+            }
+
+            // Fallback demolition in case the mass update hook did not fire; a no-op if the plot is already gone.
+            // Demolishing spawns the drops at the plot's position in the level, far away from the test structure,
+            // so the assertion has to cover the whole level. The template reset above keeps it free of stray items.
+            helper.runAtTickTime(20, () -> {
+                if (!subLevel.isRemoved()) {
+                    subLevel.getPlot().destroyAllBlocks(false);
+                }
+
+                helper.runAtTickTime(30, () -> {
+                    // Only the torches this test placed count as evidence: the shared plot grid can pop foreign
+                    // decorations from neighbouring grid content during assembly, which drops unrelated items.
+                    final List<? extends ItemEntity> droppedTorches = level.getEntities(EntityType.ITEM,
+                            itemEntity -> itemEntity.getItem().is(Blocks.WALL_TORCH.asItem()));
+                    if (!droppedTorches.isEmpty()) {
+                        helper.fail("Demolishing the invalidated plot dropped " + droppedTorches.size()
+                                + " zero-mass wall torch item(s)");
+                    }
+                    helper.succeed();
+                });
+            });
+        });
+    }
+
     @GameTest(template = "allblocks", required = false, manualOnly = true, timeoutTicks = 30_000_000)
     public static void testAllBlocks(final GameTestHelper helper) {
         final boolean failOnFirstError = false;