--- icomplete.el.orig	2020-09-01 10:14:22.000000000 +0000
+++ icomplete.el	2020-09-19 03:20:09.000000000 +0000
@@ -62,6 +62,8 @@
   :type 'string
   :version "24.4")
 
+(defcustom icomplete-vertical nil "...")
+
 (defcustom icomplete-hide-common-prefix t
   "When non-nil, hide common prefix from completion candidates.
 When nil, show candidates in full."
@@ -107,6 +109,8 @@
   :type 'integer
   :version "26.1")
 
+(defvar icomplete--prospects-actual-height 0 "...")
+
 (defcustom icomplete-compute-delay .3
   "Completions-computation stall, used only with large-number completions.
 See `icomplete-delay-completions-threshold'."
@@ -431,6 +435,13 @@
   "Run in minibuffer on activation to establish incremental completion.
 Usually run by inclusion in `minibuffer-setup-hook'."
   (when (and icomplete-mode (icomplete-simple-completing-p))
+    (setq icomplete--prospects-actual-height
+	  (min icomplete-prospects-height
+	       (1- (floor (/ (* (if (floatp max-mini-window-height)
+				    (frame-native-height)
+				  (frame-char-height))
+				max-mini-window-height)
+			     (line-pixel-height))))))
     (set (make-local-variable 'completion-show-inline-help) nil)
     (use-local-map (make-composed-keymap icomplete-minibuffer-map
     					 (current-local-map)))
@@ -650,18 +661,22 @@
 				(t (concat ellipsis (substring most compare))))
 			       close-bracket)))
 	     ;;"-prospects" - more than one candidate
-	     (prospects-len (+ (string-width
-				(or determ (concat open-bracket close-bracket)))
-			       (string-width icomplete-separator)
-			       (+ 2 (string-width ellipsis)) ;; take {…} into account
-			       (string-width (buffer-string))))
+	     (prospects-len (if icomplete-vertical
+				0
+			      (+ (string-width
+				  (or determ (concat open-bracket close-bracket)))
+				 (string-width icomplete-separator)
+				 (+ 2 (string-width ellipsis)) ;; take {…} into account
+				 (string-width (buffer-string)))))
              (prospects-max
-              ;; Max total length to use, including the minibuffer content.
-              (* (+ icomplete-prospects-height
-                    ;; If the minibuffer content already uses up more than
-                    ;; one line, increase the allowable space accordingly.
-                    (/ prospects-len (window-width)))
-                 (window-width)))
+	      (if icomplete-vertical
+		  (- icomplete--prospects-actual-height (/ (point) (window-width)))
+		;; Max total length to use, including the minibuffer content.
+		(* (+ icomplete-prospects-height
+                      ;; If the minibuffer content already uses up more than
+                      ;; one line, increase the allowable space accordingly.
+                      (/ prospects-len (window-width)))
+                   (window-width))))
              ;; Find the common prefix among `comps'.
              ;; We can't use the optimization below because its assumptions
              ;; aren't always true, e.g. when completion-cycling (bug#10850):
@@ -705,12 +720,18 @@
 	    (setq comp
 		  (if prefix-len (substring (car comps) prefix-len) (car comps))
 		  comps (cdr comps))
-	    (setq prospects-len
-                  (+ (string-width comp)
-		     (string-width icomplete-separator)
-		     prospects-len))
+	    (if icomplete-vertical
+		(if (> (length comp) 0)
+		    (setq prospects-len (1+ prospects-len)))
+	      (setq prospects-len
+                    (+ (string-width comp)
+		       (string-width icomplete-separator)
+		       prospects-len)))
 	    (if (< prospects-len prospects-max)
-		(push comp prospects)
+		(if icomplete-vertical
+		    (if (> (length comp) 0)
+			(push comp prospects))
+		  (push comp prospects))
 	      (setq limit t))))
 	(setq prospects (nreverse prospects))
         ;; Return the first match if the user hits enter.
@@ -726,11 +747,16 @@
         ;; is cached.
         (if last (setcdr last base-size))
 	(if prospects
-	    (concat determ
-		    "{"
-		    (mapconcat 'identity prospects icomplete-separator)
-		    (and limit (concat icomplete-separator ellipsis))
-		    "}")
+	    (if icomplete-vertical
+		(concat determ
+			" \n"
+			(mapconcat 'identity prospects "\n")
+			(and limit (concat "\n" ellipsis)))
+	      (concat determ
+		      "{"
+		      (mapconcat 'identity prospects icomplete-separator)
+		      (and limit (concat icomplete-separator ellipsis))
+		      "}"))
 	  (concat determ " [Matched]"))))))
 
 ;;; Iswitchb compatibility