- changed P_ChangeSector so that for bridges it only keeps the floorz and ceilingz of the spawn position.

This is necessary to prevent moving sectors from altering the bridge's z-position. The bridge should remain at its current z, even if the sector change would cause floorz or ceilingz to be changed in a way that would make P_ZMovement adjust the bridge.

src/p_map.cpp

@@ -5466,6 +5466,8 @@ bool P_AdjustFloorCeil(AActor *thing, FChangePosition *cpos)
 	}
 
 	bool isgood = P_CheckPosition(thing, thing->Pos(), tm);
+	if (!(thing->flags4 & MF4_ACTLIKEBRIDGE))
+	{
 		thing->floorz = tm.floorz;
 		thing->ceilingz = tm.ceilingz;
 		thing->dropoffz = tm.dropoffz;		// killough 11/98: remember dropoffs
@@ -5474,6 +5476,15 @@ bool P_AdjustFloorCeil(AActor *thing, FChangePosition *cpos)
 		thing->floorsector = tm.floorsector;
 		thing->ceilingpic = tm.ceilingpic;
 		thing->ceilingsector = tm.ceilingsector;
+	}
+	else
+	{
+		// Bridges only keep the info at their spawn position
+		// This is necessary to prevent moving sectors from altering the bridge's z-position.
+		// The bridge should remain at its current z, even if the sector change would cause
+		// floorz or ceilingz to be changed in a way that would make P_ZMovement adjust the bridge.
+		P_FindFloorCeiling(thing, FFCF_ONLYSPAWNPOS);
+	}
 
 	// restore the PASSMOBJ flag but leave the other flags alone.
 	thing->flags2 = (thing->flags2 & ~MF2_PASSMOBJ) | flags2;