diff --git a/src/display.c b/src/display.c
index 6486fc6d2..ebb44e872 100644
--- a/src/display.c
+++ b/src/display.c
@@ -1,4 +1,4 @@
-/* NetHack 3.7	display.c	$NHDT-Date: 1682758082 2023/04/29 08:48:02 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.216 $ */
+/* NetHack 3.7	display.c	$NHDT-Date: 1707462961 2024/02/09 07:16:01 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.231 $ */
 /* Copyright (c) Dean Luick, with acknowledgements to Kevin Darcy */
 /* and Dave Cohrs, 1990.                                          */
 /* NetHack may be freely redistributed.  See license for details. */
@@ -128,7 +128,7 @@ static void display_monster(coordxy, coordxy,
                             struct monst *, int, boolean) NONNULLPTRS;
 static int swallow_to_glyph(int, int);
 static void display_warning(struct monst *) NONNULLARG1;
-static boolean next_to_gas(struct monst *) NONNULLARG1;
+static boolean next_to_gas(struct monst *, coordxy, coordxy) NONNULLARG1;
 
 static int check_pos(coordxy, coordxy, int);
 static void get_bkglyph_and_framecolor(coordxy x, coordxy y, int *, uint32 *);
@@ -654,13 +654,18 @@ warning_of(struct monst *mon)
 /* returns True if mon is adjacent and would be seen if vision wasn't
    blocked by being in a gas cloud (implicit; caller has already checked) */
 static boolean
-next_to_gas(struct monst *mon)
+next_to_gas(
+    struct monst *mon,
+    coordxy mx, coordxy my) /* won't match mon->mx,my if long worm's tail */
 {
-    int r = (u.xray_range > 1) ? (u.xray_range + 1) : 2;
+    int r = (u.xray_range > 1) ? u.xray_range : 1;
 
-    if (distu(mon->mx, mon->my) > r * (r - 1))
+    if (distu(mx, my) > r * (r + 1))
         return FALSE;
-    if (!_mon_visible(mon) && !_see_with_infrared(mon))
+    /* decide whether monster at <mx,my> could be seen without couldsee()
+       because the gas cloud inhibits that (don't need to check infravision
+       when monster is adjacent) */
+    if (Blind || !_mon_visible(mon))
         return FALSE;
     return TRUE;
 }
@@ -938,10 +943,8 @@ newsym(coordxy x, coordxy y)
          */
         if (reg && (ACCESSIBLE(lev->typ)
                     || (reg->visible && is_pool_or_lava(x, y)))) {
-            if (mon && !worm_tail && (sensemon(mon) || mon_warning(mon)
-                                      || next_to_gas(mon))) {
-                ; /* skip region; the 'if' is more comphrehensible this way */
-            } else {
+            if (!(mon && (((sensemon(mon) || mon_warning(mon)) && !worm_tail)
+                          || next_to_gas(mon, x, y)))) { /* even if tail */
                 show_region(reg, x, y);
                 return;
             }
diff --git a/src/region.c b/src/region.c
index dac0bd116..15028e808 100644
--- a/src/region.c
+++ b/src/region.c
@@ -1,4 +1,4 @@
-/* NetHack 3.7	region.c	$NHDT-Date: 1706272460 2024/01/26 12:34:20 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.88 $ */
+/* NetHack 3.7	region.c	$NHDT-Date: 1707462965 2024/02/09 07:16:05 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.89 $ */
 /* Copyright (c) 1996 by Jean-Christophe Collet  */
 /* NetHack may be freely redistributed.  See license for details. */
 
@@ -161,6 +161,14 @@ add_mon_to_reg(NhRegion *reg, struct monst *mon)
     int i;
     unsigned *tmp_m;
 
+    /* if this is a long worm, it might already be present in the region;
+       only include it once no matter how segments the region contains */
+    if (mon_in_region(reg, mon)) {
+        if (mon->data != &mons[PM_LONG_WORM])
+            impossible("add_mon_to_reg: %s [#%u] already in region.",
+                       m_monnam(mon), mon->m_id);
+        return;
+    }
     if (reg->max_monst <= reg->n_monst) {
         tmp_m = (unsigned *) alloc(sizeof (unsigned)
                                    * (reg->max_monst + MONST_INC));
@@ -290,13 +298,27 @@ add_region(NhRegion *reg)
     /* Check for monsters inside the region */
     for (i = reg->bounding_box.lx; i <= reg->bounding_box.hx; i++)
         for (j = reg->bounding_box.ly; j <= reg->bounding_box.hy; j++) {
-            boolean is_inside = inside_region(reg, i, j);
+            struct monst *mtmp;
+            boolean is_inside = FALSE;
 
             /* Some regions can cross the level boundaries */
             if (!isok(i, j))
                 continue;
-            if (is_inside && MON_AT(i, j))
-                add_mon_to_reg(reg, gl.level.monsters[i][j]);
+            if (inside_region(reg, i, j)) {
+                is_inside = TRUE;
+                /* if there's a monster here, add it to the region */
+                if ((mtmp = m_at(i, j)) != 0
+#if 0
+                    /* leave this bit (to exclude long worm tails) out;
+                       assume that worms use "cutaneous respiration" (they
+                       breath through their skin rather than nose/gills/&c)
+                       so their tails are susceptible to poison gas */
+                    && mtmp->mx == i && mtmp->my == j
+#endif
+                    ) {
+                    add_mon_to_reg(reg, mtmp);
+                }
+            }
             if (reg->visible) {
                 if (is_inside)
                     block_point(i, j);