why wont this work...

Source/Editing/DragVerticesMode.cs

@@ -29,6 +29,7 @@ using CodeImp.DoomBuilder.IO;
 using CodeImp.DoomBuilder.Map;
 using CodeImp.DoomBuilder.Rendering;
 using CodeImp.DoomBuilder.Geometry;
+using System.Drawing;
 
 #endregion
 
@@ -225,6 +226,7 @@ namespace CodeImp.DoomBuilder.Editing
 			ICollection<Linedef> movinglines;
 			ICollection<Linedef> fixedlines;
 			ICollection<Linedef> changedlines;
+			Rectangle editarea;
 			int stitches = 0;
 			int stitchundo;
 			
@@ -245,6 +247,11 @@ namespace CodeImp.DoomBuilder.Editing
 
 				// ===== BEGIN GEOMETRY STITCHING
 				
+				// Determine area in which we are editing
+				editarea = MapSet.AreaFromVertices(selectedverts);
+				editarea.Inflate((int)Math.Ceiling(General.Settings.StitchDistance),
+					             (int)Math.Ceiling(General.Settings.StitchDistance));
+				
 				// Make undo for the stitching
 				stitchundo = General.Map.UndoRedo.CreateUndo("stitch geometry", UndoGroup.None, 0, false);
 
@@ -261,9 +268,11 @@ namespace CodeImp.DoomBuilder.Editing
 				General.Map.Map.Update();
 				
 				// Split moving lines with unselected vertices
+				unselectedverts = MapSet.FilterArea(unselectedverts, ref editarea);
 				stitches += MapSet.SplitLinesByVertices(movinglines, unselectedverts, General.Settings.StitchDistance, movinglines);
 				
 				// Split non-moving lines with selected vertices
+				fixedlines = MapSet.FilterArea(fixedlines, ref editarea);
 				stitches += MapSet.SplitLinesByVertices(fixedlines, selectedverts, General.Settings.StitchDistance, movinglines);
 
 				// Remove looped linedefs

Source/Map/Linedef.cs

@@ -24,6 +24,7 @@ using System.Text;
 using CodeImp.DoomBuilder.Geometry;
 using CodeImp.DoomBuilder.Rendering;
 using SlimDX.Direct3D;
+using System.Drawing;
 
 #endregion
 
@@ -64,7 +65,8 @@ namespace CodeImp.DoomBuilder.Map
 		private float length;
 		private float lengthinv;
 		private float angle;
-
+		private Rectangle rect;
+		
 		// Properties
 		private int flags;
 		private int action;
@@ -99,6 +101,7 @@ namespace CodeImp.DoomBuilder.Map
 		public float LengthInv { get { return lengthinv; } }
 		public float Angle { get { return angle; } }
 		public int AngleDeg { get { return (int)(angle * Angle2D.PIDEG); } }
+		public Rectangle Rect { get { return rect; } }
 
 		#endregion
 
@@ -239,6 +242,7 @@ namespace CodeImp.DoomBuilder.Map
 		public void Update()
 		{
 			Vector2D delta;
+			int l, t, r, b;
 			
 			// Update if needed
 			if(updateneeded)
@@ -251,6 +255,11 @@ namespace CodeImp.DoomBuilder.Map
 				length = (float)Math.Sqrt(lengthsq);
 				if(length > 0f) lengthinv = 1f / length; else lengthinv = 1f / 0.0000000001f;
 				angle = delta.GetAngle();
+				l = Math.Min(start.X, end.X);
+				t = Math.Min(start.Y, end.Y);
+				r = Math.Max(start.X, end.X);
+				b = Math.Max(start.Y, end.Y);
+				rect = new Rectangle(l, t, r - l, b - t);
 				
 				// Updated
 				updateneeded = false;

Source/Map/MapSet.cs

@@ -386,7 +386,87 @@ namespace CodeImp.DoomBuilder.Map
 
 		#endregion
 
-		#region ================== Static Tools
+		#region ================== Areas
+
+		// This creates an area from vertices
+		public static Rectangle AreaFromVertices(ICollection<Vertex> verts)
+		{
+			int l = int.MaxValue;
+			int t = int.MaxValue;
+			int r = int.MinValue;
+			int b = int.MinValue;
+
+			// Go for all vertices
+			foreach(Vertex v in verts)
+			{
+				// Adjust boundaries by vertices
+				if(v.X < l) l = v.X;
+				if(v.X > r) r = v.X;
+				if(v.Y < t) t = v.Y;
+				if(v.Y > b) b = v.Y;
+			}
+
+			// Return a rect
+			return new Rectangle(l, t, r - l, b - t);
+		}
+		
+		// This filters lines by a square area
+		public static ICollection<Linedef> FilterArea(ICollection<Linedef> lines, ref Rectangle area)
+		{
+			ICollection<Linedef> newlines = new List<Linedef>(lines.Count);
+			
+			// Go for all lines
+			foreach(Linedef l in lines)
+			{
+				// Check the cs field bits
+				if((GetCSFieldBits(l.Start, ref area) & GetCSFieldBits(l.End, ref area)) != 0)
+				{
+					// The line could be in the area
+					newlines.Add(l);
+				}
+			}
+			
+			// Return result
+			return newlines;
+		}
+
+		// This returns the cohen-sutherland field bits for a vertex in a rectangle area
+		private static byte GetCSFieldBits(Vertex v, ref Rectangle area)
+		{
+			byte bits = 0;
+			if(v.Y < area.Top) bits |= 0x01;
+			if(v.Y > area.Bottom) bits |= 0x02;
+			if(v.X < area.Left) bits |= 0x04;
+			if(v.X > area.Right) bits |= 0x08;
+			return bits;
+		}
+
+		// This filters vertices by a square area
+		public static ICollection<Vertex> FilterArea(ICollection<Vertex> verts, ref Rectangle area)
+		{
+			ICollection<Vertex> newverts = new List<Vertex>(verts.Count);
+
+			// Go for all verts
+			foreach(Vertex v in verts)
+			{
+				// Within rect?
+				if((v.X >= area.Left) &&
+				   (v.X <= area.Right) &&
+				   (v.Y >= area.Top) &&
+				   (v.Y <= area.Bottom))
+				{
+					// The vertex is in the area
+					newverts.Add(v);
+				}
+			}
+
+			// Return result
+			return newverts;
+		}
+
+		#endregion
+
+		#region ================== Geometry Tools
 
 		// This joins overlapping lines together
 		// Returns the number of joins made
@@ -596,9 +676,9 @@ namespace CodeImp.DoomBuilder.Map
 								nl.Update();
 
 								// Add both lines to changedlines
-								changedlines.Add(l);
-								changedlines.Add(nl);
-								
+								if(changedlines != null) changedlines.Add(l);
+								if(changedlines != null) changedlines.Add(nl);
+
 								// Count the split
 								splitsdone++;
 								splitted = true;
@@ -762,7 +842,7 @@ namespace CodeImp.DoomBuilder.Map
 		// A line is unstable when one vertex is selected and the other isn't.
 		public ICollection<Linedef> LinedefsFromSelectedVertices(bool includeunselected, bool includestable, bool includeunstable)
 		{
-			List<Linedef> list = new List<Linedef>();
+			List<Linedef> list = new List<Linedef>((linedefs.Count / 2) + 1);
 			
 			// Go for all lines
 			foreach(Linedef l in linedefs)