diff --git a/lisp/window.el b/lisp/window.el
index eba888a89d..9333f03822 100644
--- a/lisp/window.el
+++ b/lisp/window.el
@@ -9385,14 +9385,15 @@ fit-frame-to-buffer-1
              ((numberp (nth 1 sizes)) (* (nth 1 sizes) line-height))
              (t (window-min-size window nil nil t))))
            (max-width
-            (min
-             (cond
-              ((numberp max-width) (* max-width char-width))
-              ((numberp (nth 2 sizes)) (* (nth 2 sizes) char-width))
-              (t parent-or-display-width))
-             ;; The following is the maximum width that fits into the
-             ;; left and right margins.
-             (max (- right-margin left-margin outer-minus-body-width))))
+            (unless (eq only 'vertically)
+              (min
+               (cond
+                ((numberp max-width) (* max-width char-width))
+                ((numberp (nth 2 sizes)) (* (nth 2 sizes) char-width))
+                (t parent-or-display-width))
+               ;; The following is the maximum width that fits into the
+               ;; left and right margins.
+               (max (- right-margin left-margin outer-minus-body-width)))))
            (min-width
             (cond
              ((numberp min-width) (* min-width char-width))