Fixed: in some cases lines with only back side were created after dragging map elements when using "Merge Dragger Vertices Only" drag mode.

Fixed: in some cases invalid sectors were created after dragging map elements when using "Merge Dragged Geometry" and "Replace with Dragged Geometry" drag modes, when at least one of dragged linedef was facing into an enclosed void area.
Fixed a crash after drawing lines over a sector with unclosed/invalid boundaries.
Fixed(?): probably fixed a crash when closing Script Editor with an unsaved file during program termination.

Source/Core/Geometry/Tools.cs

@@ -2269,11 +2269,14 @@ namespace CodeImp.DoomBuilder.Geometry
 
 		private static bool SectorWasInvalid(Sector s)
 		{
-			if(s.Sidedefs.Count < 3 || s.FlatVertices.Length < 3)
+			if(s.Sidedefs == null || s.Sidedefs.Count < 3 || s.FlatVertices.Length < 3)
 			{
 				// Collect changed lines
 				HashSet<Linedef> changedlines = new HashSet<Linedef>();
+				if(s.Sidedefs != null)
+				{
 					foreach(Sidedef side in s.Sidedefs) changedlines.Add(side.Line);
+				}
 
 				// Delete sector
 				s.Dispose();
@@ -2400,24 +2403,26 @@ namespace CodeImp.DoomBuilder.Geometry
 		}
 
 		//mxd
-		public static Sector FindPotentialSector(Linedef line, bool front)
+		public static Sector FindSectorContaining(Linedef line, bool front)
 		{
 			List<LinedefSide> sectorsides = FindPotentialSectorAt(line, front);
 			if(sectorsides == null) return null;
 			Sector result = null;
+			bool foundstartline = false;
 
-			// Proceed only if all sectorsides reference the same sector
+			// Proceed only if all sectorsides reference the same sector and the start line is among them
 			foreach(LinedefSide sectorside in sectorsides)
 			{
 				Sidedef target = (sectorside.Front ? sectorside.Line.Front : sectorside.Line.Back);
-				if(target != null)
+				if(target == null) return null; // Fial...
-				{
+				
 				if(result == null) result = target.Sector;
 				else if(result != target.Sector) return null; // Fial...
-				}
+
+				if(sectorside.Line == line) foundstartline = true;
 			}
 
-			return result;
+			return (foundstartline ? result : null);
 		}
 
 		#endregion

Source/Core/Map/MapSet.cs

@@ -2134,12 +2134,6 @@ namespace CodeImp.DoomBuilder.Map
 
 			EndAddRemove();
 
-			//mxd. Correct sector references
-			if(mergemode != MergeGeometryMode.CLASSIC)
-			{
-				// Linedefs cache needs to be up to date...
-				Update(true, false);
-
 			// Collect changed lines... We need those in by-vertex-index order
 			// (otherwise SectorBuilder logic in some cases will incorrectly assign sector propertes)
 			List<Vertex> markedverts = GetMarkedVertices(true);
@@ -2157,6 +2151,12 @@ namespace CodeImp.DoomBuilder.Map
 				}
 			}
 
+			//mxd. Correct sector references
+			if(mergemode != MergeGeometryMode.CLASSIC)
+			{
+				// Linedefs cache needs to be up to date...
+				Update(true, false);
+				
 				// Fix stuff...
 				CorrectSectorReferences(changedlines, true);
 				CorrectOuterSides(new HashSet<Linedef>(changedlines));
@@ -2166,6 +2166,10 @@ namespace CodeImp.DoomBuilder.Map
 				HashSet<Sector> changedsectors = GetSectorsFromLinedefs(changedlines);
 				foreach(Sector s in changedsectors) s.Marked = true;
 			}
+			else
+			{
+				FlipBackwardLinedefs(changedlines);
+			}
 			
 			return true;
 		}
@@ -2437,7 +2441,7 @@ namespace CodeImp.DoomBuilder.Map
 			foreach(Linedef line in linesmissingfront)
 			{
 				// Line is now inside a sector? (check from the missing side!)
-				Sector nearest = Tools.FindPotentialSector(line, true);
+				Sector nearest = Tools.FindSectorContaining(line, true);
 
 				// We can reattach our line!
 				if(nearest != null) linefrontsectorref[line] = nearest;
@@ -2447,7 +2451,7 @@ namespace CodeImp.DoomBuilder.Map
 			foreach(Linedef line in linesmissingback)
 			{
 				// Line is now inside a sector? (check from the missing side!)
-				Sector nearest = Tools.FindPotentialSector(line, false);
+				Sector nearest = Tools.FindSectorContaining(line, false);
 
 				// We can reattach our line!
 				if(nearest != null) linebacksectorref[line] = nearest;

Source/Core/VisualModes/VisualMode.cs

@@ -515,7 +515,7 @@ namespace CodeImp.DoomBuilder.VisualModes
 		// This preforms visibility culling
 		protected void DoCulling()
 		{
-			Dictionary<Linedef, Linedef> visiblelines = new Dictionary<Linedef, Linedef>(200);
+			HashSet<Linedef> visiblelines = new HashSet<Linedef>();
 			Vector2D campos2d = General.Map.VisualCamera.Position;
 			
 			// Make collections
@@ -535,10 +535,10 @@ namespace CodeImp.DoomBuilder.VisualModes
 					foreach(Linedef ld in block.Lines)
 					{
 						// Line not already processed?
-						if(!visiblelines.ContainsKey(ld))
+						if(!visiblelines.Contains(ld))
 						{
 							// Add line if not added yet
-							visiblelines.Add(ld, ld);
+							visiblelines.Add(ld);
 
 							// Which side of the line is the camera on?
 							if(ld.SideOfLine(campos2d) < 0)
@@ -572,12 +572,12 @@ namespace CodeImp.DoomBuilder.VisualModes
 						{
 							// Create new visual thing
 							vt = CreateVisualThing(t);
-							allthings.Add(t, vt);
+							allthings[t] = vt;
 						}
 
 						if(vt != null && !visiblethings.ContainsKey(vt.Thing))
 						{
-                            visiblethings.Add(vt.Thing, vt);
+							visiblethings[vt.Thing] = vt;
 						}
 					}
 				}
@@ -586,7 +586,7 @@ namespace CodeImp.DoomBuilder.VisualModes
 			if(processgeometry)
 			{
 				// Find camera sector
-				Linedef nld = MapSet.NearestLinedef(visiblelines.Values, campos2d);
+				Linedef nld = MapSet.NearestLinedef(visiblelines, campos2d);
 				if(nld != null)
 				{
 					General.Map.VisualCamera.Sector = GetCameraSectorFromLinedef(nld);

Source/Core/Windows/ScriptEditorForm.cs

@@ -96,7 +96,7 @@ namespace CodeImp.DoomBuilder.Windows
 			editor.SaveSettings();
 			
 			// Only when closed by the user
-			if(!appclose)
+			if(!appclose && (e.CloseReason == CloseReason.UserClosing || e.CloseReason == CloseReason.FormOwnerClosing))
 			{
 				// Remember if scipts are changed
 				General.Map.ApplyScriptChanged();