diff --git a/src/p_map.c b/src/p_map.c
index 1434dd960..d8c48fa4f 100644
--- a/src/p_map.c
+++ b/src/p_map.c
@@ -1153,7 +1153,7 @@ static boolean PIT_CheckLine(line_t *ld)
 	}
 
 	// set openrange, opentop, openbottom
-	P_LineOpening(ld);
+	P_LineOpening(ld, tmthing);
 
 	// adjust floor / ceiling heights
 	if (opentop < tmceilingz)
@@ -2585,7 +2585,7 @@ static boolean PTR_SlideTraverse(intercept_t *in)
 	}
 
 	// set openrange, opentop, openbottom
-	P_LineOpening(li);
+	P_LineOpening(li, slidemo);
 
 	if (openrange < slidemo->height)
 		goto isblocking; // doesn't fit
diff --git a/src/p_maputl.c b/src/p_maputl.c
index c3a6fa842..fea8530a1 100644
--- a/src/p_maputl.c
+++ b/src/p_maputl.c
@@ -489,7 +489,7 @@ void P_CameraLineOpening(line_t *linedef)
 	}
 }
 
-void P_LineOpening(line_t *linedef)
+void P_LineOpening(line_t *linedef, mobj_t *mobj)
 {
 	sector_t *front, *back;
 
@@ -520,8 +520,8 @@ void P_LineOpening(line_t *linedef)
 	{ // Set open and high/low values here
 		fixed_t frontheight, backheight;
 
-		frontheight = P_GetCeilingZ(tmthing, front, tmx, tmy, linedef);
-		backheight = P_GetCeilingZ(tmthing, back, tmx, tmy, linedef);
+		frontheight = P_GetCeilingZ(mobj, front, tmx, tmy, linedef);
+		backheight = P_GetCeilingZ(mobj, back, tmx, tmy, linedef);
 
 		if (frontheight < backheight)
 		{
@@ -540,8 +540,8 @@ void P_LineOpening(line_t *linedef)
 #endif
 		}
 
-		frontheight = P_GetFloorZ(tmthing, front, tmx, tmy, linedef);
-		backheight = P_GetFloorZ(tmthing, back, tmx, tmy, linedef);
+		frontheight = P_GetFloorZ(mobj, front, tmx, tmy, linedef);
+		backheight = P_GetFloorZ(mobj, back, tmx, tmy, linedef);
 
 		if (frontheight > backheight)
 		{
@@ -561,12 +561,14 @@ void P_LineOpening(line_t *linedef)
 		}
 	}
 
-	if (tmthing)
+	if (mobj)
 	{
-		fixed_t thingtop = tmthing->z + tmthing->height;
+		fixed_t thingtop = mobj->z + mobj->height;
 
 		// Check for collision with front side's midtexture if Effect 4 is set
-		if (linedef->flags & ML_EFFECT4) {
+		if (linedef->flags & ML_EFFECT4
+			&& !linedef->polyobj // don't do anything for polyobjects! ...for now
+			) {
 			side_t *side = &sides[linedef->sidenum[0]];
 			fixed_t textop, texbottom, texheight;
 			fixed_t texmid, delta1, delta2;
@@ -575,30 +577,38 @@ void P_LineOpening(line_t *linedef)
 			texheight = textures[texturetranslation[side->midtexture]]->height << FRACBITS;
 
 			// Set texbottom and textop to the Z coordinates of the texture's boundaries
-#ifdef POLYOBJECTS
+#if 0 // #ifdef POLYOBJECTS
+			// don't remove this code unless solid midtextures
+			// on non-solid polyobjects should NEVER happen in the future
 			if (linedef->polyobj && (linedef->polyobj->flags & POF_TESTHEIGHT)) {
-				if (linedef->flags & ML_DONTPEGBOTTOM) {
+				if (linedef->flags & ML_EFFECT5 && !side->repeatcnt) { // "infinite" repeat
+					texbottom = back->floorheight + side->rowoffset;
+					textop = back->ceilingheight + side->rowoffset;
+				} else if (!!(linedef->flags & ML_DONTPEGBOTTOM) ^ !!(linedef->flags & ML_EFFECT3)) {
 					texbottom = back->floorheight + side->rowoffset;
 					textop = texbottom + texheight*(side->repeatcnt+1);
 				} else {
-					textop = back->ceilingheight - side->rowoffset;
+					textop = back->ceilingheight + side->rowoffset;
 					texbottom = textop - texheight*(side->repeatcnt+1);
 				}
 			} else
 #endif
 			{
-				if (linedef->flags & ML_DONTPEGBOTTOM) {
+				if (linedef->flags & ML_EFFECT5 && !side->repeatcnt) { // "infinite" repeat
+					texbottom = openbottom + side->rowoffset;
+					textop = opentop + side->rowoffset;
+				} else if (!!(linedef->flags & ML_DONTPEGBOTTOM) ^ !!(linedef->flags & ML_EFFECT3)) {
 					texbottom = openbottom + side->rowoffset;
 					textop = texbottom + texheight*(side->repeatcnt+1);
 				} else {
-					textop = opentop - side->rowoffset;
+					textop = opentop + side->rowoffset;
 					texbottom = textop - texheight*(side->repeatcnt+1);
 				}
 			}
 
 			texmid = texbottom+(textop-texbottom)/2;
 
-			delta1 = abs(tmthing->z - texmid);
+			delta1 = abs(mobj->z - texmid);
 			delta2 = abs(thingtop - texmid);
 
 			if (delta1 > delta2) { // Below
@@ -636,16 +646,16 @@ void P_LineOpening(line_t *linedef)
 				if (!(rover->flags & FF_EXISTS))
 					continue;
 
-				if (tmthing->player && (P_CheckSolidLava(tmthing, rover) || P_CanRunOnWater(tmthing->player, rover)))
+				if (mobj->player && (P_CheckSolidLava(mobj, rover) || P_CanRunOnWater(mobj->player, rover)))
 					;
-				else if (!((rover->flags & FF_BLOCKPLAYER && tmthing->player)
-					|| (rover->flags & FF_BLOCKOTHERS && !tmthing->player)))
+				else if (!((rover->flags & FF_BLOCKPLAYER && mobj->player)
+					|| (rover->flags & FF_BLOCKOTHERS && !mobj->player)))
 					continue;
 
-				topheight = P_GetFOFTopZ(tmthing, front, rover, tmx, tmy, linedef);
-				bottomheight = P_GetFOFBottomZ(tmthing, front, rover, tmx, tmy, linedef);
+				topheight = P_GetFOFTopZ(mobj, front, rover, tmx, tmy, linedef);
+				bottomheight = P_GetFOFBottomZ(mobj, front, rover, tmx, tmy, linedef);
 
-				delta1 = abs(tmthing->z - (bottomheight + ((topheight - bottomheight)/2)));
+				delta1 = abs(mobj->z - (bottomheight + ((topheight - bottomheight)/2)));
 				delta2 = abs(thingtop - (bottomheight + ((topheight - bottomheight)/2)));
 
 				if (delta1 >= delta2 && !(rover->flags & FF_PLATFORM)) // thing is below FOF
@@ -680,16 +690,16 @@ void P_LineOpening(line_t *linedef)
 				if (!(rover->flags & FF_EXISTS))
 					continue;
 
-				if (tmthing->player && (P_CheckSolidLava(tmthing, rover) || P_CanRunOnWater(tmthing->player, rover)))
+				if (mobj->player && (P_CheckSolidLava(mobj, rover) || P_CanRunOnWater(mobj->player, rover)))
 					;
-				else if (!((rover->flags & FF_BLOCKPLAYER && tmthing->player)
-					|| (rover->flags & FF_BLOCKOTHERS && !tmthing->player)))
+				else if (!((rover->flags & FF_BLOCKPLAYER && mobj->player)
+					|| (rover->flags & FF_BLOCKOTHERS && !mobj->player)))
 					continue;
 
-				topheight = P_GetFOFTopZ(tmthing, back, rover, tmx, tmy, linedef);
-				bottomheight = P_GetFOFBottomZ(tmthing, back, rover, tmx, tmy, linedef);
+				topheight = P_GetFOFTopZ(mobj, back, rover, tmx, tmy, linedef);
+				bottomheight = P_GetFOFBottomZ(mobj, back, rover, tmx, tmy, linedef);
 
-				delta1 = abs(tmthing->z - (bottomheight + ((topheight - bottomheight)/2)));
+				delta1 = abs(mobj->z - (bottomheight + ((topheight - bottomheight)/2)));
 				delta2 = abs(thingtop - (bottomheight + ((topheight - bottomheight)/2)));
 
 				if (delta1 >= delta2 && !(rover->flags & FF_PLATFORM)) // thing is below FOF
@@ -723,7 +733,7 @@ void P_LineOpening(line_t *linedef)
 			{
 				const sector_t *polysec = linedef->backsector;
 
-				delta1 = abs(tmthing->z - (polysec->floorheight + ((polysec->ceilingheight - polysec->floorheight)/2)));
+				delta1 = abs(mobj->z - (polysec->floorheight + ((polysec->ceilingheight - polysec->floorheight)/2)));
 				delta2 = abs(thingtop - (polysec->floorheight + ((polysec->ceilingheight - polysec->floorheight)/2)));
 				if (polysec->floorheight < lowestceiling && delta1 >= delta2) {
 					lowestceiling = polysec->floorheight;
diff --git a/src/p_maputl.h b/src/p_maputl.h
index c160bfa28..3d74e927b 100644
--- a/src/p_maputl.h
+++ b/src/p_maputl.h
@@ -59,7 +59,7 @@ extern fixed_t opentop, openbottom, openrange, lowfloor, highceiling;
 extern pslope_t *opentopslope, *openbottomslope;
 #endif
 
-void P_LineOpening(line_t *plinedef);
+void P_LineOpening(line_t *plinedef, mobj_t *mobj);
 
 boolean P_BlockLinesIterator(INT32 x, INT32 y, boolean(*func)(line_t *));
 boolean P_BlockThingsIterator(INT32 x, INT32 y, boolean(*func)(mobj_t *));