diff --git a/src/rendering/hwrenderer/scene/hw_bsp.cpp b/src/rendering/hwrenderer/scene/hw_bsp.cpp
index 488638203..e37b768df 100644
--- a/src/rendering/hwrenderer/scene/hw_bsp.cpp
+++ b/src/rendering/hwrenderer/scene/hw_bsp.cpp
@@ -147,10 +147,16 @@ void HWDrawInfo::WorkerThread()
 			{
 				auto portal = seg->linedef->getPortal();
 				backsector = portal->mDestination->frontsector;
+				back = hw_FakeFlat(backsector, in_area, true);
+				if (front->floorplane.isSlope() || front->ceilingplane.isSlope() || back->floorplane.isSlope() || back->ceilingplane.isSlope())
+				{
+					// Having a one-sided portal like this with slopes is too messy so let's ignore that case.
+					back = nullptr;
+				}
 			}
-			if (backsector)
+			else if (backsector)
 			{
-				if (front->sectornum == backsector->sectornum || ((seg->sidedef->Flags & WALLF_POLYOBJ) && !seg->linedef->isVisualPortal()))
+				if (front->sectornum == backsector->sectornum || (seg->sidedef->Flags & WALLF_POLYOBJ))
 				{
 					back = front;
 				}
diff --git a/src/rendering/hwrenderer/scene/hw_drawstructs.h b/src/rendering/hwrenderer/scene/hw_drawstructs.h
index 113c0aaac..063be282a 100644
--- a/src/rendering/hwrenderer/scene/hw_drawstructs.h
+++ b/src/rendering/hwrenderer/scene/hw_drawstructs.h
@@ -234,7 +234,7 @@ public:
 					  sector_t * front, sector_t * back,
 					  sector_t * realfront, sector_t * realback,
 					  float fch1, float fch2, float ffh1, float ffh2,
-					  float bch1, float bch2, float bfh1, float bfh2);
+					  float bch1, float bch2, float bfh1, float bfh2, float zalign);
 
 	void GetPlanePos(F3DFloor::planeref * planeref, float & left, float & right);
 
diff --git a/src/rendering/hwrenderer/scene/hw_walls.cpp b/src/rendering/hwrenderer/scene/hw_walls.cpp
index 6ae94fc96..4abf8d33d 100644
--- a/src/rendering/hwrenderer/scene/hw_walls.cpp
+++ b/src/rendering/hwrenderer/scene/hw_walls.cpp
@@ -1251,7 +1251,7 @@ void HWWall::DoMidTexture(HWDrawInfo *di, seg_t * seg, bool drawfogboundary,
 						  sector_t * front, sector_t * back,
 						  sector_t * realfront, sector_t * realback,
 						  float fch1, float fch2, float ffh1, float ffh2,
-						  float bch1, float bch2, float bfh1, float bfh2)
+						  float bch1, float bch2, float bfh1, float bfh2, float zalign)
 								
 {
 	FTexCoordInfo tci;
@@ -1282,12 +1282,12 @@ void HWWall::DoMidTexture(HWDrawInfo *di, seg_t * seg, bool drawfogboundary,
 		rowoffset = tci.RowOffset(seg->sidedef->GetTextureYOffset(side_t::mid));
 		if ((seg->linedef->flags & ML_DONTPEGBOTTOM) >0)
 		{
-			texturebottom = MAX(realfront->GetPlaneTexZ(sector_t::floor), realback->GetPlaneTexZ(sector_t::floor)) + rowoffset;
+			texturebottom = MAX(realfront->GetPlaneTexZ(sector_t::floor), realback->GetPlaneTexZ(sector_t::floor) + zalign) + rowoffset;
 			texturetop = texturebottom + tci.mRenderHeight;
 		}
 		else
 		{
-			texturetop = MIN(realfront->GetPlaneTexZ(sector_t::ceiling), realback->GetPlaneTexZ(sector_t::ceiling)) + rowoffset;
+			texturetop = MIN(realfront->GetPlaneTexZ(sector_t::ceiling), realback->GetPlaneTexZ(sector_t::ceiling) + zalign) + rowoffset;
 			texturebottom = texturetop - tci.mRenderHeight;
 		}
 	}
@@ -2091,9 +2091,42 @@ void HWWall::Process(HWDrawInfo *di, seg_t *seg, sector_t * frontsector, sector_
 		float bfh2 = segback->floorplane.ZatPoint(v2);
 		float bch1 = segback->ceilingplane.ZatPoint(v1);
 		float bch2 = segback->ceilingplane.ZatPoint(v2);
+		float zalign = 0.f;
 
-		SkyTop(di, seg, frontsector, backsector, v1, v2);
-		SkyBottom(di, seg, frontsector, backsector, v1, v2);
+
+		if (isportal && seg->backsector == nullptr)
+		{
+			SkyNormal(di, frontsector, v1, v2); // For sky rendering purposes this needs to be treated as a one-sided wall.
+
+			// If this is a one-sided portal and we got floor or ceiling alignment, the upper/lower texture position needs to be adjusted for that.
+			// (We assume that this portal won't involve slopes!)
+			switch (seg->linedef->getPortalAlignment())
+			{
+			case PORG_FLOOR:
+				zalign = ffh1 - bfh1;
+				bch1 += zalign;
+				bch2 += zalign;
+				bfh1 += zalign;
+				bfh2 += zalign;
+				return;
+
+			case PORG_CEILING:
+				zalign = fch1 - bch1;
+				bch1 += zalign;
+				bch2 += zalign;
+				bfh1 += zalign;
+				bfh2 += zalign;
+				return;
+
+			default:
+				break;
+			}
+		}
+		else
+		{
+			SkyTop(di, seg, frontsector, backsector, v1, v2);
+			SkyBottom(di, seg, frontsector, backsector, v1, v2);
+		}
 
 		// upper texture
 		if (frontsector->GetTexture(sector_t::ceiling) != skyflatnum || backsector->GetTexture(sector_t::ceiling) != skyflatnum)
@@ -2174,7 +2207,7 @@ void HWWall::Process(HWDrawInfo *di, seg_t *seg, sector_t * frontsector, sector_
 			if (texture && seg->backsector != nullptr)
 			{
 				DoMidTexture(di, seg, drawfogboundary, frontsector, backsector, realfront, realback,
-					fch1, fch2, ffh1, ffh2, bch1, bch2, bfh1, bfh2);
+					fch1, fch2, ffh1, ffh2, bch1, bch2, bfh1, bfh2, zalign);
 			}
 		}
 		else
@@ -2182,7 +2215,7 @@ void HWWall::Process(HWDrawInfo *di, seg_t *seg, sector_t * frontsector, sector_
 			if (texture || drawfogboundary)
 			{
 				DoMidTexture(di, seg, drawfogboundary, frontsector, backsector, realfront, realback,
-					fch1, fch2, ffh1, ffh2, bch1, bch2, bfh1, bfh2);
+					fch1, fch2, ffh1, ffh2, bch1, bch2, bfh1, bfh2, zalign);
 			}
 
 			if (backsector->e->XFloor.ffloors.Size() || frontsector->e->XFloor.ffloors.Size())