- this hack is needed because dragging vertices around can randomly create wall intersection.

In clear English: Geometry in Build maps is always broken by design. :(

source/core/gamefuncs.cpp

@@ -400,6 +400,7 @@ void dragpoint(walltype* startwall, int newx, int newy)
 	vertexscan(startwall, [&](walltype* wal)
 	{
 		wal->move(newx, newy);
+		wal->sectorp()->exflags |= SECTOREX_DRAGGED;
 	});
 }
 

source/core/maptypes.h

@@ -85,6 +85,7 @@ DEFINE_TFLAGS_OPERATORS(ESectorFlags)
 enum ESectorExBits
 {
 	SECTOREX_CLOUDSCROLL			= 1,
+	SECTOREX_DRAGGED				= 2,
 };
 
 // Flags for retriangulation

source/core/rendering/scene/hw_bunchdrawer.cpp

@@ -250,10 +250,11 @@ int BunchDrawer::ClipLine(int aline, bool portal)
 	// check against the maximum possible viewing range of the sector.
 	// Todo: check if this is sufficient or if we really have to do a more costly check against the single visible segments.
 	// Note: These walls may be excluded from the clipper, but not from being drawn!
+	// if sectors got dragged around there may be overlaps which this code does not handle well do it may not run on such sectors.
 	bool dontclip = false;
-	if (sectStartAngle != -1)
+	if (sectStartAngle != -1 && !(wall[line].sectorp()->exflags & SECTOREX_DRAGGED)) 
 	{
-		if (sectStartAngle > endAngle || sectEndAngle < startAngle)
+		if ((sectStartAngle > endAngle || sectEndAngle < startAngle))
 		{
 			dontclip = true;
 		}