Add checks in P_Move()

The `z` movement in `P_Move()` would seemingly not account for the possibility of things being over/under other things. This commit appears to fix that.

src/p_enemy.c

@@ -407,11 +407,25 @@ static boolean P_Move(mobj_t *actor, boolean dropoff) // killough 9/12/98
 
       if (actor->flags & MF_FLOAT && floatok)
         {
+          const mobj_t *onmo; // [Nugget]
+
           if (actor->z < tmfloorz)          // must adjust height
-            actor->z += FLOATSPEED;
+          {
+              actor->z += FLOATSPEED;
+
+              // [Nugget]
+              if ((onmo = P_CheckOnmobj(actor)))
+              { actor->z = onmo->z - actor->height; }
+          }
           else
+          {
             actor->z -= FLOATSPEED;
 
+            // [Nugget]
+            if ((onmo = P_CheckOnmobj(actor)))
+            { actor->z = onmo->z + onmo->height; }
+          }
+
           actor->flags |= MF_INFLOAT;
 
 	  return true;

src/p_map.c

@@ -2548,7 +2548,9 @@ mobj_t *P_CheckOnmobj(mobj_t *thing)
 {
   int xl, xh, yl, yh, bx, by;
   subsector_t *newsubsec;
-  mobj_t oldmo = *thing; // Save the old mobj before the fake movement
+  const mobj_t oldmo = *thing; // Save the old mobj before the fake movement
+
+  if (!(casual_play && over_under)) { return NULL; }
 
   tmx = thing->x;
   tmy = thing->y;