Fixed: in some cases sidedefs were incorrectly reassigned when applying Edit Selection and Drag Geometry modes.

Source/Core/Geometry/Tools.cs

@@ -2359,22 +2359,94 @@ namespace CodeImp.DoomBuilder.Geometry
 				}
 			}
 
-			// Check single-sided lines. Add new sidedefs if necessary
+			// These steps must be done in 2 phases, otherwise we'll end up getting sidedefs modified in a previous adjustment loop step
+
+			// Find sidedefs to join singlesided lines
+			Dictionary<Linedef, Sidedef> linenearestsideref = new Dictionary<Linedef, Sidedef>();
 			foreach(Linedef line in singlesidedlines)
 			{
 				// Line is now inside a sector?
 				Vector2D testpoint = line.GetSidePoint(line.Front == null);
-				Linedef nl = General.Map.Map.NearestLinedef(testpoint, selectedlines);
+				Sector nearest = General.Map.Map.GetSectorByCoordinates(testpoint, selectedsectors);
+
+				if(nearest != null)
+				{
+					Linedef nl = null;
+					float distance = float.MaxValue;
+
+					// Find nearest linedef
+					foreach(Sidedef ns in nearest.Sidedefs)
+					{
+						float d = ns.Line.SafeDistanceToSq(testpoint, true);
+						if(d < distance)
+						{
+							// This one is closer
+							nl = ns.Line;
+							distance = d;
+						}
+					}
+
+					// Find nearest sidedef
 					if(nl != null)
 					{
 						Sidedef ns = (nl.SideOfLine(testpoint) <= 0 ? nl.Front : nl.Back);
-					if(ns != null)
+						if(ns != null) linenearestsideref[line] = ns;
+					}
+				}
+			}
+
+			// Find sidedefs to join outer sides
+			Dictionary<Sidedef, Sidedef> sidenearestsideref = new Dictionary<Sidedef, Sidedef>();
+			foreach(Sidedef side in outersides)
 			{
+				// Side is inside a sector?
+				Vector2D testpoint = side.Line.GetSidePoint(side.IsFront);
+				Sector nearest = General.Map.Map.GetSectorByCoordinates(testpoint, selectedsectors);
+				sidenearestsideref[side] = null; // This side will be removed in phase 2
+
+				if(nearest != null)
+				{
+					Linedef nl = null;
+					float distance = float.MaxValue;
+
+					// Find nearest linedef
+					foreach(Sidedef ns in nearest.Sidedefs)
+					{
+						float d = ns.Line.SafeDistanceToSq(testpoint, true);
+						if(d < distance)
+						{
+							// This one is closer
+							nl = ns.Line;
+							distance = d;
+						}
+					}
+
+					// Find nearest sidedef
+					if(nl != null)
+					{
+						Sidedef ns = (nl.SideOfLine(testpoint) <= 0 ? nl.Front : nl.Back);
+						if(ns != null && ns.Sector != null)
+						{
+							if(side.Sector != ns.Sector) 
+								sidenearestsideref[side] = ns; // This side will be reattached in phase 2 
+							else
+								sidenearestsideref.Remove(side); // This side is already attached where it needs to be
+						}
+					}
+				}
+			}
+
+			// Check single-sided lines. Add new sidedefs if necessary
+			// Key is dragged single-sided line, value is the nearset side of a sector dragged line ended up in.
+			foreach(KeyValuePair<Linedef, Sidedef> group in linenearestsideref)
+			{
+				Linedef line = group.Key;
+
 				// Create new sidedef
-						Sidedef newside = General.Map.Map.CreateSidedef(line, line.Front == null, ns.Sector);
+				Sidedef newside = General.Map.Map.CreateSidedef(line, line.Front == null, group.Value.Sector);
 
 				// Copy props from the other side
-						Sidedef propssource = ((line.Front ?? line.Back) ?? ns);
+				Sidedef propssource = ((line.Front ?? line.Back) ?? group.Value);
 				propssource.CopyPropertiesTo(newside);
 				newside.RemoveUnneededTextures(true, true, true);
 				newside.Other.RemoveUnneededTextures(true, true, true);
@@ -2389,37 +2461,16 @@ namespace CodeImp.DoomBuilder.Geometry
 				// Correct the sided flags
 				line.ApplySidedFlags();
 			}
-				}
-			}
 
 			// Check outer sidedefs. Remove/change sector if necessary
-			foreach(Sidedef side in outersides)
+			// Key is outer sidedef of dragged geometry, value is nearset side of a sector dragged side ended up in.
+			foreach(KeyValuePair<Sidedef, Sidedef> group in sidenearestsideref)
 			{
-				// Side is inside a sector?
+				if(group.Value == null)
-				Vector2D testpoint = side.Line.GetSidePoint(side.IsFront);
-				Linedef nl = General.Map.Map.NearestLinedef(testpoint, selectedlines);
-				bool lineisoutside = true;
-				if(nl != null)
 				{
-					Sidedef ns = (nl.SideOfLine(testpoint) <= 0 ? nl.Front : nl.Back);
-					if(ns != null)
-					{
-						lineisoutside = false;
-						if(ns.Sector != null && side.Sector != ns.Sector)
-						{
-							// Reattach side
-							side.SetSector(ns.Sector);
-							side.RemoveUnneededTextures(true, true, true);
-						}
-					}
-				}
-
 					// Side points nowhere. Remove it
-				if(lineisoutside)
+					Linedef l = group.Key.Line;
-				{
+					group.Key.Dispose();
-					// Remove the sidedef
-					Linedef l = side.Line;
-					side.Dispose();
 
 					// Correct the linedef
 					if((l.Front == null) && (l.Back != null))
@@ -2431,6 +2482,12 @@ namespace CodeImp.DoomBuilder.Geometry
 					// Correct the sided flags
 					l.ApplySidedFlags();
 				}
+				else
+				{
+					// Reattach side
+					group.Key.SetSector(group.Value.Sector);
+					group.Key.RemoveUnneededTextures(true, true, true);
+				}
 			}
 
 			// Update map geometry

Source/Core/Map/MapSet.cs

@@ -3093,6 +3093,17 @@ namespace CodeImp.DoomBuilder.Map
 			return null;
 		}
 
+		//mxd
+		/// <summary>This returns a sector if given coordinates are inside one.</summary>
+		public Sector GetSectorByCoordinates(Vector2D pos, HashSet<Sector> sectorsToExclude)
+		{
+			foreach(Sector s in sectors)
+			{
+				if(!sectorsToExclude.Contains(s) && s.Intersect(pos)) return s;
+			}
+			return null;
+		}
+
 		//mxd
 		/// <summary>This returns a sector if given coordinates are inside one.</summary>
 		public Sector GetSectorByCoordinates(Vector2D pos, VisualBlockMap blockmap) 

Source/Plugins/BuilderModes/ClassicModes/EditSelectionMode.cs

@@ -1482,6 +1482,12 @@ namespace CodeImp.DoomBuilder.BuilderModes
 					General.Map.Map.RemoveVirtualSectors();
 				}
 
+				// Stitch geometry
+				if(snaptonearest) General.Map.Map.StitchGeometry();
+
+				// Make corrections for backward linedefs
+				MapSet.FlipBackwardLinedefs(General.Map.Map.Linedefs);
+
 				//mxd. Reattach/add/remove sidedefs only when there are no unstable lines in selection
 				if(unstablelines.Count == 0)
 				{
@@ -1491,12 +1497,6 @@ namespace CodeImp.DoomBuilder.BuilderModes
 					Tools.AdjustOuterSidedefs(affectedsectors, new HashSet<Linedef>(selectedlines));
 				}
 				
-				// Stitch geometry
-				if(snaptonearest) General.Map.Map.StitchGeometry();
-
-				// Make corrections for backward linedefs
-				MapSet.FlipBackwardLinedefs(General.Map.Map.Linedefs);
-				
 				// Snap to map format accuracy
 				General.Map.Map.SnapAllToAccuracy(General.Map.UDMF && usepreciseposition);