transient--show: Fix window height for more setups

It is desirable to set `window-size-fixed' to t in the transient menu,
as we always used to do, until very recently.

We cannot do this when the transient window uses the full height of the
frame, the minibuffer may be used while the menu remains displayed, and
the height of the minibuffer has to be increased so that the completion
framework can display completion candidates.

If `transient-hide-during-minibuffer-read' is t [1], then that cannot
happen, so we fix the size in that case again.

[1] Which is the default, starting with the parent of this commit.

Re #532.

lisp/transient.el

@@ -211,6 +211,11 @@ buffer became the current buffer, then that would change what is
 at point.  To that effect `inhibit-same-window' ensures that the
 selected window is not used to show the transient buffer.
 
+The use of a horizonal split to display the menu window can lead
+to incompatibilities and is thus discouraged.  Transient tries to
+mitigate such issue but cannot proactively deal with all possible
+configurations and combinations of third-party packages.
+
 It may be possible to display the window in another frame, but
 whether that works in practice depends on the window-manager.
 If the window manager selects the new window (Emacs frame),
@@ -3740,7 +3745,14 @@ have a history of their own.")
       (erase-buffer)
       (when transient-force-fixed-pitch
         (transient--force-fixed-pitch))
-      (setq window-size-fixed (if (window-full-height-p) 'width t))
+      (setq window-size-fixed
+            ;; If necessary, make sure the height of the minibuffer
+            ;; can be increased to display completion candidates.
+            ;; See https://github.com/minad/vertico/issues/532.
+            (if (and (not transient-hide-during-minibuffer-read)
+                     (window-full-height-p))
+                'width
+              t))
       (when (bound-and-true-p tab-line-format)
         (setq tab-line-format nil))
       (setq header-line-format nil)