Do eager display of *Completions* while idle

Don't block user input while rendering the *Completions* buffer
due to eager-display.  This allows eager-display to be used with
larger and slower completion tables without interfering with the
user.  Like in eager-update, we use while-no-input and
non-essential to ensure that eager-display happens without
blocking the user.

To support this, we remove the ability to set eager-display to a
function.  The only user was tmm.el, which nows sets
eager-display to t and adds a completion-setup-hook instead.
(This also fixes a bug in tmm where dismissing and redisplaying
the *Completions* buffer would not have the special help text)

* lisp/minibuffer.el (completion-eager-display--timer)
(completions--eager-display, completions--start-eager-display):
Add. (bug#79819)
(completing-read-default): Call
completions--start-eager-display, stop supporting functionp
eager-display.
* lisp/tmm.el (tmm-add-prompt): Delete.
(tmm--completion-setup-hook): Add.
(tmm-add-prompt): Add completion-setup-hook, set eager-display
to t.
(tmm-goto-completions): Call minibuffer-completion-help instead
of tmm-add-prompt.

lisp/minibuffer.el

@@ -141,8 +141,7 @@ This metadata is an alist.  Currently understood keys are:
 - `cycle-sort-function': function to sort entries when cycling.
    Works like `display-sort-function'.
 - `eager-display': non-nil to request eager display of the
-  completion candidates.  Can also be a function which is invoked
-  after minibuffer setup.
+  completion candidates.
 The metadata of a completion table should be constant between two boundaries."
   (let ((metadata (if (functionp table)
                       (funcall table string pred 'metadata))))
@@ -1278,7 +1277,7 @@ an association list that can specify properties such as:
 - `group-function': function for grouping the completion candidates.
 - `annotation-function': function to add annotations in *Completions*.
 - `affixation-function': function to prepend/append a prefix/suffix.
-- `eager-display': function to show *Completions* eagerly.
+- `eager-display': non-nil to show *Completions* eagerly.
 
 Categories are symbols such as `buffer' and `file', used when
 completing buffer and file names, respectively.
@@ -1300,7 +1299,7 @@ possible values are the same as in `completions-sort'.
 - `group-function': function for grouping the completion candidates.
 - `annotation-function': function to add annotations in *Completions*.
 - `affixation-function': function to prepend/append a prefix/suffix.
-- `eager-display': function to show *Completions* eagerly.
+- `eager-display': non-nil to show *Completions* eagerly.
 See more description of metadata in `completion-metadata'.
 
 Categories are symbols such as `buffer' and `file', used when
@@ -2754,6 +2753,24 @@ so that the update is less likely to interfere with user typing."
        ((completion--eager-update-p (minibuffer-prompt-end))
         (minibuffer-completion-help))))))
 
+(defvar completion-eager-display--timer nil)
+
+(defun completions--eager-display ()
+  "Try to display *Completions* without blocking input."
+  ;; If the user has left the minibuffer, give up on eager display of
+  ;; *Completions*.
+  (when (minibufferp nil t)
+    (when (while-no-input
+            (let ((non-essential t))
+              (minibuffer-completion-help)))
+      ;; If we got interrupted, try again the next time the user is idle.
+      (completions--start-eager-display))))
+
+(defun completions--start-eager-display ()
+  "Display the *Completions* buffer when the user is next idle."
+  (setq completion-eager-display--timer
+        (run-with-idle-timer 0 nil #'completions--eager-display)))
+
 (defun completions--post-command-update ()
   "Update displayed *Completions* buffer after command, once."
   (remove-hook 'post-command-hook #'completions--post-command-update)
@@ -5143,14 +5160,14 @@ See `completing-read' for the meaning of the arguments."
                 ;; `completion-eager-display' is t or if eager display
                 ;; has been requested by the completion table.
                 (when completion-eager-display
-                  (let* ((md (completion-metadata
+                  (when (or (eq completion-eager-display t)
+                            (completion-metadata-get
+                             (completion-metadata
                               (buffer-substring-no-properties
                                (minibuffer-prompt-end) (point))
-                              collection predicate))
-                         (fun (completion-metadata-get md 'eager-display)))
-                    (when (or fun (eq completion-eager-display t))
-                      (funcall (if (functionp fun)
-                                   fun #'minibuffer-completion-help))))))
+                              collection predicate)
+                             'eager-display))
+                    (completions--start-eager-display))))
             (read-from-minibuffer prompt initial-input keymap
                                   nil hist def inherit-input-method))))
     (when (and (equal result "") def)

lisp/tmm.el

@@ -218,7 +218,9 @@ is used to go back through those sub-menus."
                        (car (nth index-of-default tmm-km-list))
                      (minibuffer-with-setup-hook
                          (lambda ()
-                           (setq tmm-old-mb-map (tmm-define-keys t)))
+                           (setq tmm-old-mb-map (tmm-define-keys t))
+                           (add-hook 'completion-setup-hook
+                                     #'tmm--completion-setup-hook 'append 'local))
                        ;; tmm-km-list is reversed, because history
                        ;; needs it in LIFO order.  But default list
                        ;; needs it in non-reverse order, so that the
@@ -231,7 +233,7 @@ is used to go back through those sub-menus."
                                 " (up/down to change, PgUp to menu): ")
                         (completion-table-with-metadata
                          tmm-km-list '((category . tmm)
-                                       (eager-display . tmm-add-prompt)
+                                       (eager-display . t)
                                        (display-sort-function . identity)
                                        (cycle-sort-function . identity)))
                         nil t nil
@@ -416,20 +418,16 @@ Stores a list of all the shortcuts in the free variable `tmm-short-cuts'."
         (goto-char next)))
     (set-buffer-modified-p nil)))
 
-(defun tmm-add-prompt ()
+(defun tmm--completion-setup-hook ()
   (unless tmm-c-prompt
     (error "No active menu entries"))
   (or tmm-completion-prompt
-      (add-hook 'completion-setup-hook
-                #'tmm-completion-delete-prompt 'append))
-  (unwind-protect
-      (minibuffer-completion-help)
-    (remove-hook 'completion-setup-hook #'tmm-completion-delete-prompt))
-  (with-current-buffer "*Completions*"
+      (tmm-completion-delete-prompt))
+  (with-current-buffer standard-output
     (tmm-remove-inactive-mouse-face)
     (when tmm-completion-prompt
       (let ((inhibit-read-only t)
-	    (window (get-buffer-window "*Completions*")))
+	    (window (get-buffer-window)))
 	(goto-char (point-min))
 	(insert
          (if tmm-shortcut-inside-entry
@@ -474,7 +472,7 @@ Stores a list of all the shortcuts in the free variable `tmm-short-cuts'."
 (defun tmm-goto-completions ()
   "Jump to the completions buffer."
   (interactive)
-  (tmm-add-prompt)
+  (minibuffer-completion-help)
   (setq tmm-c-prompt (buffer-substring (minibuffer-prompt-end) (point-max)))
   ;; Clear minibuffer old contents before using *Completions* buffer for
   ;; selection.