Stinking clouds block line of sight

... you will also get a message when a seen stinking cloud
or the one surrounding the hero dissipates.

Original feature comes from Fourk, but this version (with some
minor changes) comes from xnethack by copperwater <aosdict@gmail.com>

src/vision.c

@@ -134,14 +134,14 @@ vision_init(void)
 /*
  * does_block()
  *
- * Returns true if the level feature, object, or monster at (x,y) blocks
- * sight.
+ * Returns true if something at (x,y) blocks sight.
  */
 int
 does_block(int x, int y, struct rm *lev)
 {
     struct obj *obj;
     struct monst *mon;
+    int i;
 
     /* Features that block . . */
     if (IS_ROCK(lev->typ) || lev->typ == TREE
@@ -163,6 +163,16 @@ does_block(int x, int y, struct rm *lev)
         && is_lightblocker_mappear(mon))
         return 1;
 
+    /* Clouds (poisonous or not) block light. */
+    for (i = 0; i < g.n_regions; i++) {
+        /* Ignore regions with ttl == 0 - expire_gas_cloud must unblock its
+         * points prior to being removed itself. */
+        if (g.regions[i]->ttl > 0 && g.regions[i]->visible
+            && inside_region(g.regions[i], x, y)) {
+            return 1;
+        }
+    }
+
     return 0;
 }