49 files changed, 264 insertions, 290 deletions

diff --git a/src/Blocks/BlockAnvil.h b/src/Blocks/BlockAnvil.h
index bc031216a..eb8053af1 100644
--- a/src/Blocks/BlockAnvil.h
+++ b/src/Blocks/BlockAnvil.h
@@ -2,7 +2,7 @@
 #pragma once
 
 #include "BlockHandler.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 #include "../Entities/Player.h"
 #include "../UI/AnvilWindow.h"
 
diff --git a/src/Blocks/BlockBed.h b/src/Blocks/BlockBed.h
index a6b2814b9..fcf7d58a0 100644
--- a/src/Blocks/BlockBed.h
+++ b/src/Blocks/BlockBed.h
@@ -6,7 +6,7 @@
 #include "BlockEntity.h"
 #include "ChunkInterface.h"
 #include "Entities/Player.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 
 
 
diff --git a/src/Blocks/BlockBigFlower.h b/src/Blocks/BlockBigFlower.h
index 1b2c0d735..d8270dc6c 100644
--- a/src/Blocks/BlockBigFlower.h
+++ b/src/Blocks/BlockBigFlower.h
@@ -25,9 +25,10 @@ private:
 		if (IsMetaTopPart(a_Meta))
 		{
 			BLOCKTYPE BottomType;
+			const auto BottomPosition = a_Position.addedY(-1);
 			if (
-				(a_Position.y < 1) ||
-				!a_World.GetBlockTypeMeta(a_Position - Vector3i(0, 1, 0), BottomType, a_Meta) ||
+				!cChunkDef::IsValidHeight(BottomPosition) ||
+				!a_World.GetBlockTypeMeta(BottomPosition, BottomType, a_Meta) ||
 				(BottomType != E_BLOCK_BIG_FLOWER)
 			)
 			{
@@ -104,7 +105,7 @@ private:
 		// Both parts can only that they're rooted in grass.
 
 		const auto RootPosition = a_Position.addedY(IsMetaTopPart(a_Meta) ? -2 : -1);
-		return (RootPosition.y >= 0) && IsBlockTypeOfDirt(a_Chunk.GetBlock(RootPosition));
+		return cChunkDef::IsValidHeight(RootPosition) && IsBlockTypeOfDirt(a_Chunk.GetBlock(RootPosition));
 	}
 
 
diff --git a/src/Blocks/BlockBrewingStand.h b/src/Blocks/BlockBrewingStand.h
index d1595c690..5f41525d7 100644
--- a/src/Blocks/BlockBrewingStand.h
+++ b/src/Blocks/BlockBrewingStand.h
@@ -1,7 +1,7 @@
 
 #pragma once
 
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 
 
 
diff --git a/src/Blocks/BlockButton.h b/src/Blocks/BlockButton.h
index 1a141eff3..e8affb290 100644
--- a/src/Blocks/BlockButton.h
+++ b/src/Blocks/BlockButton.h
@@ -7,7 +7,7 @@
 #include "../Chunk.h"
 #include "Defines.h"
 #include "Entities/Player.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 #include "ChunkInterface.h"
 #include "World.h"
 
diff --git a/src/Blocks/BlockCactus.h b/src/Blocks/BlockCactus.h
index 23c4d3421..9b1461cb8 100644
--- a/src/Blocks/BlockCactus.h
+++ b/src/Blocks/BlockCactus.h
@@ -20,11 +20,12 @@ private:
 
 	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
 	{
-		if (a_Position.y <= 0)
+		const auto SurfacePosition = a_Position.addedY(-1);
+		if (!cChunkDef::IsValidHeight(SurfacePosition))
 		{
 			return false;
 		}
-		BLOCKTYPE Surface = a_Chunk.GetBlock(a_Position.addedY(-1));
+		BLOCKTYPE Surface = a_Chunk.GetBlock(SurfacePosition);
 		if ((Surface != E_BLOCK_SAND) && (Surface != E_BLOCK_CACTUS))
 		{
 			// Cactus can only be placed on sand and itself
@@ -75,25 +76,25 @@ private:
 	virtual int Grow(cChunk & a_Chunk, Vector3i a_RelPos, int a_NumStages = 1) const override
 	{
 		// Check the total height of the cacti blocks here:
-		int top = a_RelPos.y + 1;
+		auto Top = a_RelPos.addedY(1);
 		while (
-			(top < cChunkDef::Height) &&
-			(a_Chunk.GetBlock({a_RelPos.x, top, a_RelPos.z}) == E_BLOCK_CACTUS)
+			cChunkDef::IsValidHeight(Top) &&
+			(a_Chunk.GetBlock(Top) == E_BLOCK_CACTUS)
 		)
 		{
-			++top;
+			Top.y++;
 		}
-		int bottom = a_RelPos.y - 1;
+		auto Bottom = a_RelPos.addedY(-1);
 		while (
-			(bottom > 0) &&
-			(a_Chunk.GetBlock({a_RelPos.x, bottom, a_RelPos.z}) == E_BLOCK_CACTUS)
+			cChunkDef::IsValidHeight(Bottom) &&
+			(a_Chunk.GetBlock(Bottom) == E_BLOCK_CACTUS)
 		)
 		{
-			--bottom;
+			--Bottom.y;
 		}
 
 		// Refuse if already too high:
-		auto numToGrow = std::min(a_NumStages, a_Chunk.GetWorld()->GetMaxCactusHeight() + 1 - (top - bottom));
+		auto numToGrow = std::min(a_NumStages, a_Chunk.GetWorld()->GetMaxCactusHeight() + 1 - (Top.y - Bottom.y));
 		if (numToGrow <= 0)
 		{
 			return 0;
@@ -102,14 +103,14 @@ private:
 		BLOCKTYPE blockType;
 		for (int i = 0; i < numToGrow; ++i)
 		{
-			Vector3i pos(a_RelPos.x, top + i, a_RelPos.z);
-			if (!a_Chunk.UnboundedRelGetBlockType(pos, blockType) || (blockType != E_BLOCK_AIR))
+			auto NewTop = Top.addedY(i);
+			if (!a_Chunk.UnboundedRelGetBlockType(NewTop, blockType) || (blockType != E_BLOCK_AIR))
 			{
 				// Cannot grow there
 				return i;
 			}
 
-			a_Chunk.UnboundedRelFastSetBlock(pos, E_BLOCK_CACTUS, 0);
+			a_Chunk.UnboundedRelFastSetBlock(NewTop, E_BLOCK_CACTUS, 0);
 
 			// Check surroundings. Cacti may ONLY be surrounded by non-solid blocks; if they aren't, drop as pickup and bail out the growing
 			static const Vector3i neighborOffsets[] =
@@ -122,7 +123,7 @@ private:
 			for (const auto & ofs: neighborOffsets)
 			{
 				if (
-					a_Chunk.UnboundedRelGetBlockType(pos + ofs, blockType) &&
+					a_Chunk.UnboundedRelGetBlockType(NewTop + ofs, blockType) &&
 					(
 						cBlockInfo::IsSolid(blockType) ||
 						(blockType == E_BLOCK_LAVA) ||
@@ -131,7 +132,7 @@ private:
 				)
 				{
 					// Remove the cactus
-					auto absPos = a_Chunk.RelativeToAbsolute(pos);
+					auto absPos = a_Chunk.RelativeToAbsolute(NewTop);
 					a_Chunk.GetWorld()->DropBlockAsPickups(absPos);
 					return i + 1;
 				}
@@ -143,7 +144,8 @@ private:
 	virtual PlantAction CanGrow(cChunk & a_Chunk, Vector3i a_RelPos) const override
 	{
 		// Only allow growing if there's an air block above:
-		if (((a_RelPos.y + 1) < cChunkDef::Height) && (a_Chunk.GetBlock(a_RelPos.addedY(1)) == E_BLOCK_AIR))
+		const auto RelPosAbove = a_RelPos.addedY(1);
+		if (cChunkDef::IsValidHeight(RelPosAbove) && (a_Chunk.GetBlock(RelPosAbove) == E_BLOCK_AIR))
 		{
 			return Super::CanGrow(a_Chunk, a_RelPos);
 		}
diff --git a/src/Blocks/BlockCarpet.h b/src/Blocks/BlockCarpet.h
index 841e94b11..2477ee344 100644
--- a/src/Blocks/BlockCarpet.h
+++ b/src/Blocks/BlockCarpet.h
@@ -27,7 +27,8 @@ private:
 
 	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
 	{
-		return (a_Position.y > 0) && (a_Chunk.GetBlock(a_Position.addedY(-1)) != E_BLOCK_AIR);
+		const auto PosBelow = a_Position.addedY(-1);
+		return cChunkDef::IsValidHeight(PosBelow) && (a_Chunk.GetBlock(PosBelow) != E_BLOCK_AIR);
 	}
 
 
diff --git a/src/Blocks/BlockChest.h b/src/Blocks/BlockChest.h
index 119216d00..99a95e692 100644
--- a/src/Blocks/BlockChest.h
+++ b/src/Blocks/BlockChest.h
@@ -3,7 +3,7 @@
 
 #include "../BlockArea.h"
 #include "../Entities/Player.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 
 
 
diff --git a/src/Blocks/BlockCocoaPod.h b/src/Blocks/BlockCocoaPod.h
index e018388c5..7906a1888 100644
--- a/src/Blocks/BlockCocoaPod.h
+++ b/src/Blocks/BlockCocoaPod.h
@@ -43,7 +43,12 @@ private:
 		auto LogPos = AddFaceDirection(a_Position, BlockFace, true);
 		BLOCKTYPE BlockType;
 		NIBBLETYPE BlockMeta;
-		a_Chunk.UnboundedRelGetBlock(LogPos, BlockType, BlockMeta);
+		if (!a_Chunk.UnboundedRelGetBlock(LogPos, BlockType, BlockMeta))
+		{
+			// Don't pop if chunk not loaded.
+			return true;
+		}
+
 		return ((BlockType == E_BLOCK_LOG) && ((BlockMeta & 0x03) == E_META_LOG_JUNGLE));
 	}
 
diff --git a/src/Blocks/BlockComparator.h b/src/Blocks/BlockComparator.h
index 18aa2a8b9..9ce9d3dbd 100644
--- a/src/Blocks/BlockComparator.h
+++ b/src/Blocks/BlockComparator.h
@@ -2,17 +2,17 @@
 #pragma once
 
 #include "BlockHandler.h"
-#include "BlockRedstoneRepeater.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
+#include "Mixins/SolidSurfaceUnderneath.h"
 
 
 
 
 
 class cBlockComparatorHandler final :
-	public cYawRotator<cBlockHandler, 0x03, 0x00, 0x01, 0x02, 0x03>
+	public cSolidSurfaceUnderneath<cYawRotator<cBlockHandler, 0x03, 0x00, 0x01, 0x02, 0x03>>
 {
-	using Super = cYawRotator<cBlockHandler, 0x03, 0x00, 0x01, 0x02, 0x03>;
+	using Super = cSolidSurfaceUnderneath<cYawRotator<cBlockHandler, 0x03, 0x00, 0x01, 0x02, 0x03>>;
 
 public:
 
@@ -152,41 +152,6 @@ private:
 
 
 
-	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
-	{
-		if (a_Position.y <= 0)
-		{
-			return false;
-		}
-
-		BLOCKTYPE BelowBlock;
-		NIBBLETYPE BelowBlockMeta;
-		a_Chunk.GetBlockTypeMeta(a_Position.addedY(-1), BelowBlock, BelowBlockMeta);
-
-		if (cBlockInfo::FullyOccupiesVoxel(BelowBlock))
-		{
-			return true;
-		}
-
-		// upside down slabs
-		if (cBlockSlabHandler::IsAnySlabType(BelowBlock))
-		{
-			return BelowBlockMeta & E_META_WOODEN_SLAB_UPSIDE_DOWN;
-		}
-
-		// upside down stairs
-		if (cBlockStairsHandler::IsAnyStairType(BelowBlock))
-		{
-			return BelowBlockMeta & E_BLOCK_STAIRS_UPSIDE_DOWN;
-		}
-
-		return false;
-	}
-
-
-
-
-
 	virtual cItems ConvertToPickups(const NIBBLETYPE a_BlockMeta, const cItem * const a_Tool) const override
 	{
 		return cItem(E_ITEM_COMPARATOR, 1, 0);
diff --git a/src/Blocks/BlockCrops.h b/src/Blocks/BlockCrops.h
index b323ace10..59413ba08 100644
--- a/src/Blocks/BlockCrops.h
+++ b/src/Blocks/BlockCrops.h
@@ -118,7 +118,14 @@ private:
 
 	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
 	{
-		return (a_Position.y > 0) && (a_Chunk.GetBlock(a_Position.addedY(-1)) == E_BLOCK_FARMLAND);
+		const auto BlockBelow = a_Position.addedY(-1);
+
+		if (!cChunkDef::IsValidHeight(BlockBelow))
+		{
+			return false;
+		}
+
+		return a_Chunk.GetBlock(BlockBelow) == E_BLOCK_FARMLAND;
 	}
 
 
diff --git a/src/Blocks/BlockDeadBush.h b/src/Blocks/BlockDeadBush.h
index 080dd150c..884b6f686 100644
--- a/src/Blocks/BlockDeadBush.h
+++ b/src/Blocks/BlockDeadBush.h
@@ -29,12 +29,13 @@ private:
 
 	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
 	{
-		if (a_Position.y <= 0)
+		const auto PosBelow = a_Position.addedY(-1);
+		if (!cChunkDef::IsValidHeight(PosBelow))
 		{
 			return false;
 		}
 
-		BLOCKTYPE BelowBlock = a_Chunk.GetBlock(a_Position.addedY(-1));
+		BLOCKTYPE BelowBlock = a_Chunk.GetBlock(PosBelow);
 		switch (BelowBlock)
 		{
 			case E_BLOCK_CLAY:
diff --git a/src/Blocks/BlockDoor.h b/src/Blocks/BlockDoor.h
index 1b5c33d46..c394bda39 100644
--- a/src/Blocks/BlockDoor.h
+++ b/src/Blocks/BlockDoor.h
@@ -5,7 +5,7 @@
 #include "../BlockInfo.h"
 #include "../Entities/Player.h"
 #include "../Chunk.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 #include "ChunkInterface.h"
 #include "BlockSlab.h"
 
@@ -191,14 +191,18 @@ private:
 	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
 	{
 		// CanBeAt is also called on placement, so the top part can't check for the bottom part.
-		// Both parts can only that their base is a valid block.
+		// Both parts can only check that the base of the door (i.e. -2 for a door top) is a valid block.
+		const auto BasePosition = a_Position.addedY(((a_Meta & 0x8) == 0x8) ? -2 : -1);
+		if (!cChunkDef::IsValidHeight(BasePosition))
+		{
+			return false;
+		}
 
 		BLOCKTYPE BlockType;
 		NIBBLETYPE BlockMeta;
-		const auto BasePosition = a_Position.addedY(((a_Meta & 0x8) == 0x8) ? -2 : -1);
 		a_Chunk.GetBlockTypeMeta(BasePosition, BlockType, BlockMeta);
 
-		return (BasePosition.y >= 0) && CanBeOn(BlockType, BlockMeta);
+		return CanBeOn(BlockType, BlockMeta);
 	}
 
 
@@ -216,9 +220,10 @@ private:
 		if ((Meta & 0x08) != 0)
 		{
 			// The coords are pointing at the top part of the door
-			if (a_BlockPos.y > 0)
+			const auto BottomPos = a_BlockPos.addedY(-1);
+			if (cChunkDef::IsValidHeight(BottomPos))
 			{
-				NIBBLETYPE DownMeta = a_ChunkInterface.GetBlockMeta(a_BlockPos.addedY(-1));
+				NIBBLETYPE DownMeta = a_ChunkInterface.GetBlockMeta(BottomPos);
 				return static_cast<NIBBLETYPE>((DownMeta & 0x07) | 0x08 | (Meta << 4));
 			}
 			// This is the top part of the door at the bottommost layer of the world, there's no bottom:
@@ -227,9 +232,10 @@ private:
 		else
 		{
 			// The coords are pointing at the bottom part of the door
-			if (a_BlockPos.y < cChunkDef::Height - 1)
+			const auto TopPos = a_BlockPos.addedY(1);
+			if (cChunkDef::IsValidHeight(TopPos))
 			{
-				NIBBLETYPE UpMeta = a_ChunkInterface.GetBlockMeta(a_BlockPos.addedY(1));
+				NIBBLETYPE UpMeta = a_ChunkInterface.GetBlockMeta(TopPos);
 				return static_cast<NIBBLETYPE>(Meta | (UpMeta << 4));
 			}
 			// This is the bottom part of the door at the topmost layer of the world, there's no top:
diff --git a/src/Blocks/BlockDropSpenser.h b/src/Blocks/BlockDropSpenser.h
index 050adf0f1..7b18eb8da 100644
--- a/src/Blocks/BlockDropSpenser.h
+++ b/src/Blocks/BlockDropSpenser.h
@@ -5,7 +5,7 @@
 
 #pragma once
 
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 
 
 
diff --git a/src/Blocks/BlockEnderChest.h b/src/Blocks/BlockEnderChest.h
index 2e0da0f17..a944ab263 100644
--- a/src/Blocks/BlockEnderChest.h
+++ b/src/Blocks/BlockEnderChest.h
@@ -1,7 +1,7 @@
 
 #pragma once
 
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 
 
 
diff --git a/src/Blocks/BlockFenceGate.h b/src/Blocks/BlockFenceGate.h
index f52d8df9c..75a0ded06 100644
--- a/src/Blocks/BlockFenceGate.h
+++ b/src/Blocks/BlockFenceGate.h
@@ -2,7 +2,7 @@
 #pragma once
 
 #include "BlockHandler.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 #include "../EffectID.h"
 
 
diff --git a/src/Blocks/BlockFlower.h b/src/Blocks/BlockFlower.h
index 023ba4a37..aaa532c0c 100644
--- a/src/Blocks/BlockFlower.h
+++ b/src/Blocks/BlockFlower.h
@@ -2,15 +2,16 @@
 #pragma once
 
 #include "BlockHandler.h"
+#include "Mixins/DirtLikeUnderneath.h"
 
 
 
 
 
 class cBlockFlowerHandler final :
-	public cBlockHandler
+	public cDirtLikeUnderneath<cBlockHandler>
 {
-	using Super = cBlockHandler;
+	using Super = cDirtLikeUnderneath<cBlockHandler>;
 
 public:
 
@@ -28,15 +29,6 @@ private:
 
 
 
-	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
-	{
-		return (a_Position.y > 0) && IsBlockTypeOfDirt(a_Chunk.GetBlock(a_Position.addedY(-1)));
-	}
-
-
-
-
-
 	virtual ColourID GetMapBaseColourID(NIBBLETYPE a_Meta) const override
 	{
 		UNUSED(a_Meta);
diff --git a/src/Blocks/BlockFurnace.h b/src/Blocks/BlockFurnace.h
index 8cdc9489a..968ea398c 100644
--- a/src/Blocks/BlockFurnace.h
+++ b/src/Blocks/BlockFurnace.h
@@ -1,7 +1,7 @@
 
 #pragma once
 
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 
 
 
diff --git a/src/Blocks/BlockGlazedTerracotta.h b/src/Blocks/BlockGlazedTerracotta.h
index 0975f1ecd..21206ab77 100644
--- a/src/Blocks/BlockGlazedTerracotta.h
+++ b/src/Blocks/BlockGlazedTerracotta.h
@@ -1,7 +1,7 @@
 
 #pragma once
 
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 
 
 
diff --git a/src/Blocks/BlockHopper.h b/src/Blocks/BlockHopper.h
index 930f1e0bc..91ad91bff 100644
--- a/src/Blocks/BlockHopper.h
+++ b/src/Blocks/BlockHopper.h
@@ -3,7 +3,7 @@
 
 // Declares the cBlockHopperHandler class representing the handler for the Hopper block
 
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 
 
 
diff --git a/src/Blocks/BlockJukebox.h b/src/Blocks/BlockJukebox.h
index 970f10fc1..79475d30b 100644
--- a/src/Blocks/BlockJukebox.h
+++ b/src/Blocks/BlockJukebox.h
@@ -2,7 +2,7 @@
 #pragma once
 
 #include "BlockEntity.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 
 
 
diff --git a/src/Blocks/BlockLadder.h b/src/Blocks/BlockLadder.h
index bab51b2d3..4bca19930 100644
--- a/src/Blocks/BlockLadder.h
+++ b/src/Blocks/BlockLadder.h
@@ -3,7 +3,7 @@
 
 #include "BlockHandler.h"
 #include "BlockInfo.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 
 
 
diff --git a/src/Blocks/BlockLever.h b/src/Blocks/BlockLever.h
index 60db4ddee..2a676ce00 100644
--- a/src/Blocks/BlockLever.h
+++ b/src/Blocks/BlockLever.h
@@ -5,7 +5,7 @@
 #include "Blocks/BlockStairs.h"
 #include "ChunkDef.h"
 #include "Defines.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 #include "BlockSlab.h"
 
 
diff --git a/src/Blocks/BlockMushroom.h b/src/Blocks/BlockMushroom.h
index bed16c557..9e4378d4e 100644
--- a/src/Blocks/BlockMushroom.h
+++ b/src/Blocks/BlockMushroom.h
@@ -23,14 +23,15 @@ private:
 
 	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
 	{
-		if (a_Position.y <= 0)
+		const auto BasePos = a_Position.addedY(-1);
+		if (!cChunkDef::IsValidHeight(BasePos))
 		{
 			return false;
 		}
 
 		// TODO: Cannot be at too much daylight
 
-		switch (a_Chunk.GetBlock(a_Position.addedY(-1)))
+		switch (a_Chunk.GetBlock(BasePos))
 		{
 			case E_BLOCK_GLASS:
 			case E_BLOCK_CACTUS:
diff --git a/src/Blocks/BlockNetherWart.h b/src/Blocks/BlockNetherWart.h
index 43081d511..c0686afc6 100644
--- a/src/Blocks/BlockNetherWart.h
+++ b/src/Blocks/BlockNetherWart.h
@@ -60,7 +60,8 @@ private:
 	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
 	{
 		// Needs to be placed on top of a Soulsand block:
-		return (a_Position.y > 0) && (a_Chunk.GetBlock(a_Position.addedY(-1)) == E_BLOCK_SOULSAND);
+		const auto BasePos = a_Position.addedY(-1);
+		return cChunkDef::IsValidHeight(BasePos) && (a_Chunk.GetBlock(BasePos) == E_BLOCK_SOULSAND);
 	}
 
 
diff --git a/src/Blocks/BlockObserver.h b/src/Blocks/BlockObserver.h
index 13ffa4631..c2cab3068 100644
--- a/src/Blocks/BlockObserver.h
+++ b/src/Blocks/BlockObserver.h
@@ -2,7 +2,7 @@
 #pragma once
 
 #include "BlockHandler.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 
 
 class cBlockObserverHandler final :
diff --git a/src/Blocks/BlockPiston.h b/src/Blocks/BlockPiston.h
index 4afeb4767..275214036 100644
--- a/src/Blocks/BlockPiston.h
+++ b/src/Blocks/BlockPiston.h
@@ -2,7 +2,7 @@
 #pragma once
 
 #include "BlockHandler.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 #include "../Item.h"
 
 
diff --git a/src/Blocks/BlockPortal.h b/src/Blocks/BlockPortal.h
index 05daa9337..f4b5a50bd 100644
--- a/src/Blocks/BlockPortal.h
+++ b/src/Blocks/BlockPortal.h
@@ -51,9 +51,9 @@ private:
 
 	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
 	{
-		if ((a_Position.y <= 0) || (a_Position.y >= cChunkDef::Height - 1))
+		if (!cChunkDef::IsValidHeight(a_Position.addedY(-1)) || !cChunkDef::IsValidHeight(a_Position.addedY(1)))
 		{
-			return false;  // In case someone places a portal with meta 1 or 2 at boundaries, and server tries to get invalid coords at Y - 1 or Y + 1.
+			return false;  // Must be 1 away from the boundary, there will always be another portal or an obsidian between the portal block and the boundary.
 		}
 
 		switch (a_Meta)
diff --git a/src/Blocks/BlockPressurePlate.h b/src/Blocks/BlockPressurePlate.h
index 6d852bfc0..d69234cee 100644
--- a/src/Blocks/BlockPressurePlate.h
+++ b/src/Blocks/BlockPressurePlate.h
@@ -22,14 +22,15 @@ private:
 
 	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
 	{
-		if (a_Position.y <= 0)
+		const auto PosBelow = a_Position.addedY(-1);
+		if (!cChunkDef::IsValidHeight(PosBelow))
 		{
 			return false;
 		}
 
 		BLOCKTYPE Block;
 		NIBBLETYPE BlockMeta;
-		a_Chunk.GetBlockTypeMeta(a_Position.addedY(-1), Block, BlockMeta);
+		a_Chunk.GetBlockTypeMeta(PosBelow, Block, BlockMeta);
 
 		// upside down slabs
 		if (cBlockSlabHandler::IsAnySlabType(Block))
diff --git a/src/Blocks/BlockPumpkin.h b/src/Blocks/BlockPumpkin.h
index e627eeaee..fa73db56a 100644
--- a/src/Blocks/BlockPumpkin.h
+++ b/src/Blocks/BlockPumpkin.h
@@ -1,7 +1,7 @@
 #pragma once
 
 #include "BlockHandler.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 
 
 
diff --git a/src/Blocks/BlockRail.h b/src/Blocks/BlockRail.h
index 4e2e6211f..dea84296f 100644
--- a/src/Blocks/BlockRail.h
+++ b/src/Blocks/BlockRail.h
@@ -1,10 +1,9 @@
 #pragma once
 
 #include "BlockHandler.h"
-#include "BlockSlab.h"
-#include "BlockStairs.h"
 #include "BlockType.h"
-#include "Blocks/Mixins.h"
+#include "Mixins/Mixins.h"
+#include "Mixins/SolidSurfaceUnderneath.h"
 #include "../BlockInfo.h"
 #include "../Chunk.h"
 #include "ChunkDef.h"
@@ -25,9 +24,9 @@ enum ENUM_PURE
 
 
 class cBlockRailHandler final :
-	public cClearMetaOnDrop<cBlockHandler>
+	public cSolidSurfaceUnderneath<cClearMetaOnDrop<cBlockHandler>>
 {
-	using Super = cClearMetaOnDrop<cBlockHandler>;
+	using Super = cSolidSurfaceUnderneath<cClearMetaOnDrop<cBlockHandler>>;
 
 public:
 
@@ -162,26 +161,9 @@ public:
 
 private:
 
-	static bool CanBeSupportedBy(BLOCKTYPE a_BlockType, NIBBLETYPE a_BlockMeta)
-	{
-		if (cBlockSlabHandler::IsAnySlabType(a_BlockType))
-		{
-			return (a_BlockMeta & E_META_WOODEN_SLAB_UPSIDE_DOWN);
-		}
-		else if (cBlockStairsHandler::IsAnyStairType(a_BlockType))
-		{
-			return (a_BlockMeta & E_BLOCK_STAIRS_UPSIDE_DOWN);
-		}
-		return cBlockInfo::FullyOccupiesVoxel(a_BlockType);
-	}
-
 	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, NIBBLETYPE a_Meta) const override
 	{
-		BLOCKTYPE BelowBlock;
-		NIBBLETYPE BelowBlockMeta;
-		a_Chunk.GetBlockTypeMeta(a_Position.addedY(-1), BelowBlock, BelowBlockMeta);
-
-		if ((a_Position.y <= 0) || !CanBeSupportedBy(BelowBlock, BelowBlockMeta))
+		if (!Super::CanBeAt(a_Chunk, a_Position, a_Meta))
 		{
 			return false;
 		}
diff --git a/src/Blocks/BlockRedstoneRepeater.h b/src/Blocks/BlockRedstoneRepeater.h
index 893691d7b..92b4b3917 100644
--- a/src/Blocks/BlockRedstoneRepeater.h
+++ b/src/Blocks/BlockRedstoneRepeater.h
@@ -3,10 +3,9 @@
 
 #include "BlockHandler.h"
 #include "BlockType.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
+#include "Mixins/SolidSurfaceUnderneath.h"
 #include "ChunkInterface.h"
-#include "BlockSlab.h"
-#include "BlockStairs.h"
 #include "../Chunk.h"
 
 
@@ -14,9 +13,9 @@
 
 
 class cBlockRedstoneRepeaterHandler final :
-	public cYawRotator<cBlockHandler, 0x03, 0x00, 0x01, 0x02, 0x03>
+	public cSolidSurfaceUnderneath<cYawRotator<cBlockHandler, 0x03, 0x00, 0x01, 0x02, 0x03>>
 {
-	using Super = cYawRotator<cBlockHandler, 0x03, 0x00, 0x01, 0x02, 0x03>;
+	using Super = cSolidSurfaceUnderneath<cYawRotator<cBlockHandler, 0x03, 0x00, 0x01, 0x02, 0x03>>;
 
 public:
 
@@ -107,41 +106,6 @@ private:
 
 
 
-	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
-	{
-		if (a_Position.y <= 0)
-		{
-			return false;
-		}
-
-		BLOCKTYPE BelowBlock;
-		NIBBLETYPE BelowBlockMeta;
-		a_Chunk.GetBlockTypeMeta(a_Position.addedY(-1), BelowBlock, BelowBlockMeta);
-
-		if (cBlockInfo::FullyOccupiesVoxel(BelowBlock))
-		{
-			return true;
-		}
-
-		// upside down slabs
-		if (cBlockSlabHandler::IsAnySlabType(BelowBlock))
-		{
-			return BelowBlockMeta & E_META_WOODEN_SLAB_UPSIDE_DOWN;
-		}
-
-		// upside down stairs
-		if (cBlockStairsHandler::IsAnyStairType(BelowBlock))
-		{
-			return BelowBlockMeta & E_BLOCK_STAIRS_UPSIDE_DOWN;
-		}
-
-		return false;
-	}
-
-
-
-
-
 	virtual cItems ConvertToPickups(const NIBBLETYPE a_BlockMeta, const cItem * const a_Tool) const override
 	{
 		return cItem(E_ITEM_REDSTONE_REPEATER, 1, 0);
diff --git a/src/Blocks/BlockRedstoneWire.h b/src/Blocks/BlockRedstoneWire.h
index de8e59a40..edea704f4 100644
--- a/src/Blocks/BlockRedstoneWire.h
+++ b/src/Blocks/BlockRedstoneWire.h
@@ -2,17 +2,15 @@
 #pragma once
 
 #include "BlockHandler.h"
-#include "BlockSlab.h"
-#include "BlockStairs.h"
-#include "../Chunk.h"
+#include "Mixins/SolidSurfaceUnderneath.h"
 
 
 
 
 class cBlockRedstoneWireHandler final :
-	public cBlockHandler
+	public cSolidSurfaceUnderneath<cBlockHandler>
 {
-	using Super = cBlockHandler;
+	using Super = cSolidSurfaceUnderneath<cBlockHandler>;
 
 public:
 
@@ -20,41 +18,6 @@ public:
 
 private:
 
-	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
-	{
-		if (a_Position.y <= 0)
-		{
-			return false;
-		}
-
-		BLOCKTYPE BelowBlock;
-		NIBBLETYPE BelowBlockMeta;
-		a_Chunk.GetBlockTypeMeta(a_Position.addedY(-1), BelowBlock, BelowBlockMeta);
-
-		if (cBlockInfo::FullyOccupiesVoxel(BelowBlock))
-		{
-			return true;
-		}
-
-		// upside down slabs
-		if (cBlockSlabHandler::IsAnySlabType(BelowBlock))
-		{
-			return BelowBlockMeta & E_META_WOODEN_SLAB_UPSIDE_DOWN;
-		}
-
-		// upside down stairs
-		if (cBlockStairsHandler::IsAnyStairType(BelowBlock))
-		{
-			return BelowBlockMeta & E_BLOCK_STAIRS_UPSIDE_DOWN;
-		}
-
-		return false;
-	}
-
-
-
-
-
 	virtual cItems ConvertToPickups(const NIBBLETYPE a_BlockMeta, const cItem * const a_Tool) const override
 	{
 		return cItem(E_ITEM_REDSTONE_DUST, 1, 0);
diff --git a/src/Blocks/BlockSapling.h b/src/Blocks/BlockSapling.h
index d32b9b449..db953946a 100644
--- a/src/Blocks/BlockSapling.h
+++ b/src/Blocks/BlockSapling.h
@@ -3,15 +3,16 @@
 
 #include "BlockHandler.h"
 #include "../FastRandom.h"
+#include "Mixins/DirtLikeUnderneath.h"
 
 
 
 
 
 class cBlockSaplingHandler final :
-	public cBlockHandler
+	public cDirtLikeUnderneath<cBlockHandler>
 {
-	using Super = cBlockHandler;
+	using Super = cDirtLikeUnderneath<cBlockHandler>;
 
 public:
 
@@ -29,15 +30,6 @@ private:
 
 
 
-	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
-	{
-		return (a_Position.y > 0) && IsBlockTypeOfDirt(a_Chunk.GetBlock(a_Position.addedY(-1)));
-	}
-
-
-
-
-
 	virtual void OnUpdate(
 		cChunkInterface & a_ChunkInterface,
 		cWorldInterface & a_WorldInterface,
diff --git a/src/Blocks/BlockSignPost.h b/src/Blocks/BlockSignPost.h
index b96498cbd..89da77fbb 100644
--- a/src/Blocks/BlockSignPost.h
+++ b/src/Blocks/BlockSignPost.h
@@ -30,12 +30,13 @@ private:
 
 	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
 	{
-		if (a_Position.y <= 0)
+		const auto BelowPos = a_Position.addedY(-1);
+		if (!cChunkDef::IsValidHeight(BelowPos))
 		{
 			return false;
 		}
 
-		BLOCKTYPE Type = a_Chunk.GetBlock(a_Position.addedY(-1));
+		BLOCKTYPE Type = a_Chunk.GetBlock(BelowPos);
 		return (Type == E_BLOCK_SIGN_POST) || (Type == E_BLOCK_WALLSIGN) || cBlockInfo::IsSolid(Type);
 	}
 
diff --git a/src/Blocks/BlockSnow.h b/src/Blocks/BlockSnow.h
index f3fa87a1b..51361ce17 100644
--- a/src/Blocks/BlockSnow.h
+++ b/src/Blocks/BlockSnow.h
@@ -69,11 +69,11 @@ private:
 
 	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
 	{
-		if (a_Position.y <= 0)
+		const auto BelowPos = a_Position.addedY(-1);
+		if (!cChunkDef::IsValidHeight(BelowPos))
 		{
 			return false;
 		}
-		auto BelowPos = a_Position.addedY(-1);
 		auto BlockBelow = a_Chunk.GetBlock(BelowPos);
 		auto MetaBelow = a_Chunk.GetMeta(BelowPos);
 		return CanBeOn(BlockBelow, MetaBelow);
diff --git a/src/Blocks/BlockStairs.h b/src/Blocks/BlockStairs.h
index 76614bb62..6c37a8464 100644
--- a/src/Blocks/BlockStairs.h
+++ b/src/Blocks/BlockStairs.h
@@ -2,7 +2,7 @@
 #pragma once
 
 #include "BlockHandler.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 
 
 
diff --git a/src/Blocks/BlockStandingBanner.h b/src/Blocks/BlockStandingBanner.h
index 391b7fde7..0cc7c28eb 100644
--- a/src/Blocks/BlockStandingBanner.h
+++ b/src/Blocks/BlockStandingBanner.h
@@ -5,15 +5,16 @@
 
 #include "../BlockInfo.h"
 #include "BlockEntity.h"
+#include "Mixins/SolidSurfaceUnderneath.h"
 
 
 
 
 
 class cBlockStandingBannerHandler final :
-	public cBlockEntityHandler
+	public cSolidSurfaceUnderneath<cBlockEntityHandler>
 {
-	using Super = cBlockEntityHandler;
+	using Super = cSolidSurfaceUnderneath<cBlockEntityHandler>;
 
 public:
 
@@ -29,20 +30,6 @@ public:
 
 
 
-	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
-	{
-		if (a_Position.y < 1)
-		{
-			return false;
-		}
-
-		return cBlockInfo::IsSolid(a_Chunk.GetBlock(a_Position.addedY(-1)));
-	}
-
-
-
-
-
 	virtual ColourID GetMapBaseColourID(NIBBLETYPE a_Meta) const override
 	{
 		UNUSED(a_Meta);
diff --git a/src/Blocks/BlockStems.h b/src/Blocks/BlockStems.h
index 11cd83967..9d598003d 100644
--- a/src/Blocks/BlockStems.h
+++ b/src/Blocks/BlockStems.h
@@ -58,7 +58,8 @@ private:
 
 	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
 	{
-		return (a_Position.y > 0) && (a_Chunk.GetBlock(a_Position.addedY(-1)) == E_BLOCK_FARMLAND);
+		const auto BelowPos = a_Position.addedY(-1);
+		return cChunkDef::IsValidHeight(BelowPos) && (a_Chunk.GetBlock(BelowPos) == E_BLOCK_FARMLAND);
 	}
 
 
diff --git a/src/Blocks/BlockSugarCane.h b/src/Blocks/BlockSugarCane.h
index cffe667e5..1f6c8a8fa 100644
--- a/src/Blocks/BlockSugarCane.h
+++ b/src/Blocks/BlockSugarCane.h
@@ -29,12 +29,13 @@ private:
 
 	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
 	{
-		if (a_Position.y <= 0)
+		const auto BelowPos = a_Position.addedY(-1);
+		if (!cChunkDef::IsValidHeight(BelowPos))
 		{
 			return false;
 		}
 
-		switch (a_Chunk.GetBlock(a_Position.addedY(-1)))
+		switch (a_Chunk.GetBlock(BelowPos))
 		{
 			case E_BLOCK_DIRT:
 			case E_BLOCK_GRASS:
@@ -43,16 +44,16 @@ private:
 			{
 				static const Vector3i Coords[] =
 				{
-					{-1, -1,  0},
-					{ 1, -1,  0},
-					{ 0, -1, -1},
-					{ 0, -1,  1},
+					{-1, 0,  0},
+					{ 1, 0,  0},
+					{ 0, 0, -1},
+					{ 0, 0,  1},
 				} ;
 				for (size_t i = 0; i < ARRAYCOUNT(Coords); i++)
 				{
 					BLOCKTYPE BlockType;
 					NIBBLETYPE BlockMeta;
-					if (!a_Chunk.UnboundedRelGetBlock(a_Position + Coords[i], BlockType, BlockMeta))
+					if (!a_Chunk.UnboundedRelGetBlock(BelowPos + Coords[i], BlockType, BlockMeta))
 					{
 						// Too close to the edge, cannot simulate
 						return true;
@@ -90,31 +91,36 @@ private:
 	virtual int Grow(cChunk & a_Chunk, Vector3i a_RelPos, int a_NumStages = 1) const override
 	{
 		// Check the total height of the sugarcane blocks here:
-		int top = a_RelPos.y + 1;
+		auto top = a_RelPos.addedY(1);
 		while (
-			(top < cChunkDef::Height) &&
-			(a_Chunk.GetBlock({a_RelPos.x, top, a_RelPos.z}) == E_BLOCK_SUGARCANE)
+			cChunkDef::IsValidHeight(top) &&
+			(a_Chunk.GetBlock(top) == E_BLOCK_SUGARCANE)
 		)
 		{
-			++top;
+			++top.y;
 		}
-		int bottom = a_RelPos.y - 1;
+		auto bottom = a_RelPos.addedY(-1);
 		while (
-			(bottom > 0) &&
-			(a_Chunk.GetBlock({a_RelPos.x, bottom, a_RelPos.z}) == E_BLOCK_SUGARCANE)
+			cChunkDef::IsValidHeight(bottom) &&
+			(a_Chunk.GetBlock(bottom) == E_BLOCK_SUGARCANE)
 		)
 		{
-			--bottom;
+			--bottom.y;
 		}
 
 		// Grow by at most a_NumStages, but no more than max height:
-		auto toGrow = std::min(a_NumStages, a_Chunk.GetWorld()->GetMaxSugarcaneHeight() + 1 - (top - bottom));
-		Vector3i topPos(a_RelPos.x, top, a_RelPos.z);
+		auto toGrow = std::min(a_NumStages, a_Chunk.GetWorld()->GetMaxSugarcaneHeight() + 1 - (top.y - bottom.y));
 		for (int i = 0; i < toGrow; i++)
 		{
-			if (a_Chunk.GetBlock(topPos.addedY(i)) == E_BLOCK_AIR)
+			const auto NewTop = top.addedY(i);
+			if (!cChunkDef::IsValidHeight(NewTop))
 			{
-				a_Chunk.SetBlock(topPos.addedY(i), E_BLOCK_SUGARCANE, 0);
+				return i;
+			}
+
+			if (a_Chunk.GetBlock(NewTop) == E_BLOCK_AIR)
+			{
+				a_Chunk.SetBlock(NewTop, E_BLOCK_SUGARCANE, 0);
 			}
 			else
 			{
diff --git a/src/Blocks/BlockTallGrass.h b/src/Blocks/BlockTallGrass.h
index ea638c878..47ad2f93d 100644
--- a/src/Blocks/BlockTallGrass.h
+++ b/src/Blocks/BlockTallGrass.h
@@ -2,6 +2,7 @@
 #pragma once
 
 #include "BlockHandler.h"
+#include "Mixins/DirtLikeUnderneath.h"
 #include "ChunkInterface.h"
 
 
@@ -10,9 +11,9 @@
 
 /** Handles the grass that is 1 block tall */
 class cBlockTallGrassHandler final :
-	public cBlockHandler
+	public cDirtLikeUnderneath<cBlockHandler>
 {
-	using Super = cBlockHandler;
+	using Super = cDirtLikeUnderneath<cBlockHandler>;
 
 public:
 
@@ -52,25 +53,11 @@ private:
 
 
 
-	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
-	{
-		if (a_Position.y <= 0)
-		{
-			return false;
-		}
-
-		BLOCKTYPE BelowBlock = a_Chunk.GetBlock(a_Position.addedY(-1));
-		return IsBlockTypeOfDirt(BelowBlock);
-	}
-
-
-
-
-
 	/** Growing a tall grass produces a big flower (2-block high fern or double-tall grass). */
 	virtual int Grow(cChunk & a_Chunk, Vector3i a_RelPos, int a_NumStages = 1) const override
 	{
-		if (a_RelPos.y > (cChunkDef::Height - 2))
+		const auto TopPos = a_RelPos.addedY(1);
+		if (!cChunkDef::IsValidHeight(TopPos))
 		{
 			return 0;
 		}
@@ -83,7 +70,7 @@ private:
 			default:                      return 0;
 		}
 		a_Chunk.SetBlock(a_RelPos,           E_BLOCK_BIG_FLOWER, largeFlowerMeta);
-		a_Chunk.SetBlock(a_RelPos.addedY(1), E_BLOCK_BIG_FLOWER, E_META_BIG_FLOWER_TOP);
+		a_Chunk.SetBlock(TopPos, E_BLOCK_BIG_FLOWER, E_META_BIG_FLOWER_TOP);
 		return 1;
 	}
 
diff --git a/src/Blocks/BlockTorch.h b/src/Blocks/BlockTorch.h
index ad69cf5eb..99f685cb6 100644
--- a/src/Blocks/BlockTorch.h
+++ b/src/Blocks/BlockTorch.h
@@ -7,7 +7,7 @@
 #include "BlockType.h"
 #include "ChunkInterface.h"
 #include "Defines.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 
 
 
@@ -126,7 +126,7 @@ private:
 		NIBBLETYPE NeighborBlockMeta;
 		if (!a_Chunk.UnboundedRelGetBlock(NeighborRelPos, NeighborBlockType, NeighborBlockMeta))
 		{
-			// Neighbor in an unloaded chunk, bail out without changint this.
+			// Neighbor in an unloaded chunk, bail out without changing this.
 			return false;
 		}
 
diff --git a/src/Blocks/BlockTrapdoor.h b/src/Blocks/BlockTrapdoor.h
index 7f6f70e1e..3fb4f4ca3 100644
--- a/src/Blocks/BlockTrapdoor.h
+++ b/src/Blocks/BlockTrapdoor.h
@@ -2,7 +2,7 @@
 #pragma once
 
 #include "BlockHandler.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 #include "../EffectID.h"
 
 
diff --git a/src/Blocks/BlockTripwireHook.h b/src/Blocks/BlockTripwireHook.h
index ae553d66f..ede6ca311 100644
--- a/src/Blocks/BlockTripwireHook.h
+++ b/src/Blocks/BlockTripwireHook.h
@@ -1,7 +1,7 @@
 #pragma once
 
 #include "BlockHandler.h"
-#include "Mixins.h"
+#include "Mixins/Mixins.h"
 
 
 
diff --git a/src/Blocks/BlockVines.h b/src/Blocks/BlockVines.h
index a23ba1489..edbd30f36 100644
--- a/src/Blocks/BlockVines.h
+++ b/src/Blocks/BlockVines.h
@@ -119,15 +119,16 @@ private:
 		}
 
 		// Check if vine above us, add its meta to MaxMeta:
-		if ((a_Position.y < cChunkDef::Height - 1) && (a_Chunk.GetBlock(a_Position.addedY(1)) == E_BLOCK_VINES))
+		const auto AbovePos = a_Position.addedY(1);
+		if (cChunkDef::IsValidHeight(AbovePos) && (a_Chunk.GetBlock(AbovePos) == E_BLOCK_VINES))
 		{
-			MaxMeta |= a_Chunk.GetMeta(a_Position.addedY(1));
+			MaxMeta |= a_Chunk.GetMeta(AbovePos);
 		}
 
 		NIBBLETYPE Common = a_CurrentMeta & MaxMeta;  // Neighbors that we have and are legal.
 		if (Common != a_CurrentMeta)
 		{
-			bool HasTop = (a_Position.y < (cChunkDef::Height - 1)) && IsBlockAttachable(a_Chunk.GetBlock(a_Position.addedY(1)));
+			bool HasTop = cChunkDef::IsValidHeight(AbovePos) && IsBlockAttachable(a_Chunk.GetBlock(AbovePos));
 			if ((Common == 0) && !HasTop)  // Meta equals 0 also means top. Make a last-ditch attempt to save the vine.
 			{
 				return VINE_LOST_SUPPORT;
diff --git a/src/Blocks/CMakeLists.txt b/src/Blocks/CMakeLists.txt
index 38a60d5b7..7e3047451 100644
--- a/src/Blocks/CMakeLists.txt
+++ b/src/Blocks/CMakeLists.txt
@@ -110,6 +110,6 @@ target_sources(
 	BroadcastInterface.h
 	ChunkInterface.h
 	GetHandlerCompileTimeTemplate.h
-	Mixins.h
+	Mixins/Mixins.h
 	WorldInterface.h
 )
diff --git a/src/Blocks/Mixins/DirtLikeUnderneath.h b/src/Blocks/Mixins/DirtLikeUnderneath.h
new file mode 100644
index 000000000..909f1601e
--- /dev/null
+++ b/src/Blocks/Mixins/DirtLikeUnderneath.h
@@ -0,0 +1,42 @@
+
+#pragma once
+
+#include "../../Chunk.h"
+
+/** Mixin to ensure the block has a dirt-like block underneath. */
+template <class Base>
+class cDirtLikeUnderneath :
+	public Base
+{
+	using Super = Base;
+public:
+
+	using Super::Super;
+
+	constexpr cDirtLikeUnderneath(BLOCKTYPE a_BlockType):
+		Base(a_BlockType)
+	{
+	}
+
+protected:
+
+	~cDirtLikeUnderneath() = default;
+
+protected:
+
+	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
+	{
+		if (!Super::CanBeAt(a_Chunk, a_Position, a_Meta))
+		{
+			return false;
+		}
+
+		const auto BelowPos = a_Position.addedY(-1);
+		if (!cChunkDef::IsValidHeight(BelowPos))
+		{
+			return false;
+		}
+
+		return IsBlockTypeOfDirt(a_Chunk.GetBlock(BelowPos));
+	}
+};
diff --git a/src/Blocks/Mixins.h b/src/Blocks/Mixins/Mixins.h
index fa8985737..d9a233bad 100644
--- a/src/Blocks/Mixins.h
+++ b/src/Blocks/Mixins/Mixins.h
@@ -9,7 +9,7 @@ class cBlockLadder: public cMetaRotator<cClearMetaOnDrop, ...>
 
 #pragma once
 
-#include "../Item.h"
+#include "../../Item.h"
 
 
 
diff --git a/src/Blocks/Mixins/SolidSurfaceUnderneath.h b/src/Blocks/Mixins/SolidSurfaceUnderneath.h
new file mode 100644
index 000000000..c54c064d6
--- /dev/null
+++ b/src/Blocks/Mixins/SolidSurfaceUnderneath.h
@@ -0,0 +1,65 @@
+
+#pragma once
+
+#include "../../Chunk.h"
+#include "../BlockSlab.h"
+#include "../BlockStairs.h"
+
+/** Mixin to ensure the block has a solid surface underneath. */
+template <class Base>
+class cSolidSurfaceUnderneath :
+	public Base
+{
+	using Super = Base;
+public:
+
+	using Super::Super;
+
+	constexpr cSolidSurfaceUnderneath(BLOCKTYPE a_BlockType):
+		Base(a_BlockType)
+	{
+	}
+
+protected:
+
+	~cSolidSurfaceUnderneath() = default;
+
+protected:
+
+	virtual bool CanBeAt(const cChunk & a_Chunk, const Vector3i a_Position, const NIBBLETYPE a_Meta) const override
+	{
+		if (!Super::CanBeAt(a_Chunk, a_Position, a_Meta))
+		{
+			return false;
+		}
+
+		const auto BelowPos = a_Position.addedY(-1);
+		if (!cChunkDef::IsValidHeight(BelowPos))
+		{
+			return false;
+		}
+
+		BLOCKTYPE BelowBlock;
+		NIBBLETYPE BelowBlockMeta;
+		a_Chunk.GetBlockTypeMeta(BelowPos, BelowBlock, BelowBlockMeta);
+
+		if (cBlockInfo::FullyOccupiesVoxel(BelowBlock))
+		{
+			return true;
+		}
+
+		// upside down slabs
+		if (cBlockSlabHandler::IsAnySlabType(BelowBlock))
+		{
+			return BelowBlockMeta & E_META_WOODEN_SLAB_UPSIDE_DOWN;
+		}
+
+		// upside down stairs
+		if (cBlockStairsHandler::IsAnyStairType(BelowBlock))
+		{
+			return BelowBlockMeta & E_BLOCK_STAIRS_UPSIDE_DOWN;
+		}
+
+		return false;
+	}
+};