Fix floor disappearing with z-levels off (#35651)

The collapse code was performing a lot of operations on neighboring
z-levels, without first checking if experimental z-levels were enabled.
This was causing bugs due to these operations being performed on the
current z-level instead.
Fixes #35266

src/map.cpp

@@ -2965,14 +2965,15 @@ void map::collapse_at( const tripoint &p, const bool silent, const bool was_supp
     // If something supporting the roof collapsed, see what else collapses
     if( supports && !still_supports ) {
         for( const tripoint &t : points_in_radius( p, 1 ) ) {
+            // If z-levels are off, tz == t, so we end up skipping a lot of stuff to avoid bugs.
             const tripoint &tz = tripoint( t.xy(), t.z + 1 );
             // if nothing above us had the chance of collapsing, move on
             if( !one_in( collapse_check( tz ) ) ) {
                 continue;
             }
             // if a wall collapses, walls without support from below risk collapsing and
             //propogate the collapse upwards
-            if( wall && p == t && has_flag( "WALL", tz ) ) {
+            if( zlevels && wall && p == t && has_flag( "WALL", tz ) ) {
                 collapse_at( tz, silent );
             }
             // floors without support from below risk collapsing into open air and can propogate
@@ -2982,8 +2983,10 @@ void map::collapse_at( const tripoint &p, const bool silent, const bool was_supp
             }
             // this tile used to support a roof, now it doesn't, which means there is only
             // open air above us
-            ter_set( tz, t_open_air );
-            furn_set( tz, f_null );
+            if( zlevels ) {
+                ter_set( tz, t_open_air );
+                furn_set( tz, f_null );
+            }
         }
     }
     // it would be great to check if collapsing ceilings smashed through the floor, but