fix: fallback to stty size when LINES env var is missing

Some environments (tmux) export COLUMNS but not LINES. Use
'stty size' as a fallback for the missing dimension.

src/backend/modern.lisp

@@ -189,6 +189,15 @@ as a fallback when a keyword is not in *named-colors*.")
                      (rstr (sb-ext:posix-getenv "LINES"))
                      (cols (when cstr (parse-integer cstr :junk-allowed t)))
                      (rows (when rstr (parse-integer rstr :junk-allowed t))))
+                ;; Some environments set only COLUMNS or only LINES.
+                ;; If one is missing, try the other from stty.
+                (when (and cols (null rows))
+                  (let* ((out (uiop:run-program '("stty" "size")
+                                                 :output :string
+                                                 :ignore-error-status t))
+                         (parts (when out (uiop:split-string (string-trim '(#\newline #\space) out)))))
+                    (setf rows (when (and parts (= (length parts) 2))
+                                 (parse-integer (second parts) :junk-allowed t)))))
                 (when (and cols rows (> cols 0) (> rows 0))
                   (values cols rows))))
             (values w h)))))