From fab38d092b5b1cc736aa2916523a12cd5823da16 Mon Sep 17 00:00:00 2001
From: Christoph Oelckers <c.oelckers@zdoom.fake>
Date: Wed, 27 Apr 2016 02:13:35 +0200
Subject: [PATCH] - fixed: P_LineOpening could miss a 3D floor if an actor was
 centered right against its side. - restored the original 3D floor code to
 retrieve the current floor in P_CheckPosition. The portal aware version was a
 bit too strict and could place the actor on the wrong side when moving at
 high speeds.

---
 src/p_3dfloors.cpp |  2 +-
 src/p_map.cpp      | 40 +++++++++++++++++++++++++++++++++++++++-
 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/src/p_3dfloors.cpp b/src/p_3dfloors.cpp
index 8da672513..5e2117b9b 100644
--- a/src/p_3dfloors.cpp
+++ b/src/p_3dfloors.cpp
@@ -790,7 +790,7 @@ void P_LineOpening_XFloors (FLineOpening &open, AActor * thing, const line_t *li
 						lowestceilingsec = j == 0 ? linedef->frontsector : linedef->backsector;
 					}
 					
-					if(ff_top > highestfloor && delta1 < delta2 && (!restrict || thing->Z() >= ff_top))
+					if(ff_top > highestfloor && delta1 <= delta2 && (!restrict || thing->Z() >= ff_top))
 					{
 						highestfloor = ff_top;
 						highestfloorpic = *rover->top.texture;
diff --git a/src/p_map.cpp b/src/p_map.cpp
index 3d04c6889..bd442fa61 100644
--- a/src/p_map.cpp
+++ b/src/p_map.cpp
@@ -1514,7 +1514,45 @@ bool P_CheckPosition(AActor *thing, const DVector2 &pos, FCheckPosition &tm, boo
 	// Any contacted lines the step closer together will adjust them.
 	if (!thing->IsNoClip2())
 	{
-		P_GetFloorCeilingZ(tm, FFCF_SAMESECTOR);
+		if (!newsec->PortalBlocksMovement(sector_t::ceiling) || !newsec->PortalBlocksMovement(sector_t::floor))
+		{
+			// Use P_GetFloorCeilingZ only if there's portals to consider. Its logic is subtly different than what is needed here for 3D floors.
+			P_GetFloorCeilingZ(tm, FFCF_SAMESECTOR);
+		}
+		else
+		{
+			tm.floorz = tm.dropoffz = newsec->floorplane.ZatPoint(pos);
+			tm.floorpic = newsec->GetTexture(sector_t::floor);
+			tm.ceilingz = newsec->ceilingplane.ZatPoint(pos);
+			tm.ceilingpic = newsec->GetTexture(sector_t::ceiling);
+			tm.floorsector = tm.ceilingsector = newsec;
+		}
+
+		F3DFloor*  rover;
+		double thingtop = thing->Height > 0 ? thing->Top() : thing->Z() + 1;
+
+		for (unsigned i = 0; i<newsec->e->XFloor.ffloors.Size(); i++)
+		{
+			rover = newsec->e->XFloor.ffloors[i];
+			if (!(rover->flags & FF_SOLID) || !(rover->flags & FF_EXISTS)) continue;
+
+			double ff_bottom = rover->bottom.plane->ZatPoint(pos);
+			double ff_top = rover->top.plane->ZatPoint(pos);
+
+			double delta1 = thing->Z() - (ff_bottom + ((ff_top - ff_bottom) / 2));
+			double delta2 = thingtop - (ff_bottom + ((ff_top - ff_bottom) / 2));
+
+			if (ff_top > tm.floorz && fabs(delta1) < fabs(delta2))
+			{
+				tm.floorz = tm.dropoffz = ff_top;
+				tm.floorpic = *rover->top.texture;
+			}
+			if (ff_bottom < tm.ceilingz && abs(delta1) >= abs(delta2))
+			{
+				tm.ceilingz = ff_bottom;
+				tm.ceilingpic = *rover->bottom.texture;
+			}
+		}
 	}
 	else
 	{