diff --git a/dotscape b/dotscape
index a40e2fc..2334bc4 100755
Binary files a/dotscape and b/dotscape differ
diff --git a/fcore/quad-tree.sml b/fcore/quad-tree.sml
index 7f0fa41..5a7db3c 100644
--- a/fcore/quad-tree.sml
+++ b/fcore/quad-tree.sml
@@ -93,7 +93,8 @@ struct
         end
     | LEAF => acc
 
-  fun toList tree = foldr (fn (item, acc) => item ::acc, tree, [])
+  fun toList tree =
+    foldr (fn (item, acc) => item :: acc, tree, [])
 end
 
 structure CollisionTree =
@@ -189,10 +190,10 @@ struct
              )
          end)
 
-    fun getClickPoint (clickPoints, pos) =
-      let val idx = Int.min (pos, Vector.length clickPoints - 1)
-      in Vector.sub (clickPoints, idx)
-      end
+  fun getClickPoint (clickPoints, pos) =
+    let val idx = Int.min (pos, Vector.length clickPoints - 1)
+    in Vector.sub (clickPoints, idx)
+    end
 
   fun folder
     ( windowWidth
@@ -313,58 +314,134 @@ struct
            NODE {tl = tl, tr = tr, bl = bl, br = br}
          end)
 
-    local
-      fun loop (x, y, ex, ey, grid) =
-        if x > 0 then
-          if quadHasSameColour (x, y, ex, ey, grid) then
-            loop (x - 1, y, x, ey, grid)
-          else
-            ex
+  local
+    fun loop (x, y, ex, ey, grid) =
+      if x > 0 then
+        if quadHasSameColour (x, y, ex, ey, grid) then
+          loop (x - 1, y, x, ey, grid)
         else
-          0
-    in
-      fun getLeftmostX ({x, y, ex, ey, data}, grid) =
-        loop (x - 1, y, x, ey, grid)
-    end
+          ex
+      else
+        0
+  in
+    fun getLeftmostX ({x, y, ex, ey, data}, grid) =
+      loop (x - 1, y, x, ey, grid)
+  end
 
-    local
-      fun loop (x, y, ex, ey, grid) =
-        if x < Vector.length grid andalso ex < Vector.length grid then
-          if quadHasSameColour (x, y, ex, ey, grid) then
-            loop (ex, y, ex + 1, ey, grid)
-          else
-            x
+  local
+    fun loop (x, y, ex, ey, grid) =
+      if x < Vector.length grid andalso ex < Vector.length grid then
+        if quadHasSameColour (x, y, ex, ey, grid) then
+          loop (ex, y, ex + 1, ey, grid)
         else
-          Vector.length grid - 1
-    in
-      fun getRightmostX ({x, y, ex, ey, data}, grid) =
-        loop (ex, y, ex + 1, ey, grid)
-    end
+          x
+      else
+        Vector.length grid - 1
+  in
+    fun getRightmostX ({x, y, ex, ey, data}, grid) =
+      loop (ex, y, ex + 1, ey, grid)
+  end
 
-    local
-      fun loop (x, y, ex, ey, grid) =
-        if y < 0 then 0
-        else
-          if quadHasSameColour (x, y, ex, ey, grid) then
-            loop (x, y - 1, ex, y, grid)
-          else
-            ey
-    in
-      fun getTopmostY ({x, y, ex, ey, data}, grid) =
+  local
+    fun loop (x, y, ex, ey, grid) =
+      if y < 0 then
+        0
+      else if quadHasSameColour (x, y, ex, ey, grid) then
         loop (x, y - 1, ex, y, grid)
-    end
+      else
+        ey
+  in
+    fun getTopmostY ({x, y, ex, ey, data}, grid) =
+      loop (x, y - 1, ex, y, grid)
+  end
 
-    local
-      fun loop (x, y, ex, ey, grid) =
-        if y < Vector.length grid andalso ey < Vector.length grid then
-          if quadHasSameColour (x, y, ex, ey, grid) then
-            loop (x, ey, ex, ey + 1, grid)
-          else
-            y
+  local
+    fun loop (x, y, ex, ey, grid) =
+      if y < Vector.length grid andalso ey < Vector.length grid then
+        if quadHasSameColour (x, y, ex, ey, grid) then
+          loop (x, ey, ex, ey + 1, grid)
         else
-          Vector.length grid - 1
+          y
+      else
+        Vector.length grid - 1
+  in
+    fun getBottomY ({x, y, ex, ey, data}, grid) =
+      loop (x, ey, ex, ey + 1, grid)
+  end
+
+  local
+    fun loop (tree, grid) =
+      case tree of
+        EMPTY => (EMPTY, false)
+      | LEAF (oldItem as {x, y, ex, ey, data}) =>
+          let
+            val topY = getTopmostY (oldItem, grid)
+            val bottomY = getBottomY (oldItem, grid)
+            val newItem = {y = topY, ey = bottomY, x = x, ex = ex, data = data}
+            val didItemChange = newItem <> oldItem
+          in
+            (LEAF newItem, didItemChange)
+          end
+      | NODE {tl, tr, bl, br} =>
+          let
+            val (tl, didTlChange) = loop (tl, grid)
+            val (tr, didTrChange) = loop (tr, grid)
+            val (bl, didBlChange) = loop (bl, grid)
+            val (br, didBrChange) = loop (br, grid)
+
+            val node = NODE {tl = tl, tr = tr, bl = bl, br = br}
+            val didChange =
+              didTlChange orelse didTrChange orelse didBlChange
+              orelse didBrChange
+          in
+            (node, didChange)
+          end
+  in
+    fun mergeVertical (tree, grid) =
+      let val (newTree, didChange) = loop (tree, grid)
+      in if didChange then mergeVertical (newTree, grid) else newTree
+      end
+  end
+
+  local
+    fun loop (tree, grid) =
+      case tree of
+        EMPTY => (EMPTY, false)
+      | LEAF (oldItem as {x, y, ex, ey, data}) =>
+          let
+            val leftX = getLeftmostX (oldItem, grid)
+            val rightX = getRightmostX (oldItem, grid)
+            val newItem = {x = leftX, ex = rightX, y = y, ey = ey, data = data}
+            val didItemChange = newItem <> oldItem
+          in
+            (LEAF newItem, didItemChange)
+          end
+      | NODE {tl, tr, bl, br} =>
+          let
+            val (tl, didTlChange) = loop (tl, grid)
+            val (tr, didTrChange) = loop (tr, grid)
+            val (bl, didBlChange) = loop (bl, grid)
+            val (br, didBrChange) = loop (br, grid)
+
+            val node = NODE {tl = tl, tr = tr, bl = bl, br = br}
+            val didChange =
+              didTlChange orelse didTrChange orelse didBlChange
+              orelse didBrChange
+          in
+            (node, didChange)
+          end
+  in
+    fun mergeHorizontal (tree, grid) =
+      let val (newTree, didChange) = loop (tree, grid)
+      in if didChange then mergeHorizontal (newTree, grid) else newTree
+      end
+  end
+
+  fun merge (tree, grid) =
+    let
+      val tree = mergeVertical (tree, grid)
+      val tree = mergeHorizontal (tree, grid)
     in
-      fun getBottomY ({x, y, ex, ey, data}, grid) =
-        loop (x, ey, ex, ey + 1, grid)
+      BinTree.toList tree
     end
 end