transient-prefix: Add display-action slot

CHANGELOG

@@ -26,6 +26,10 @@
 - ~transient-setup-buffer-hook~ is now run later to allow overriding
   more default settings.
 
+- The new prefix slots ~display-action~ and ~mode-line-format~, can be
+  used to override ~transient-display-buffer-action~ and
+  ~transient-mode-line-format~ for individual prefix menus.  #332
+
 Bug fixes:
 
 - Fixes some menu navigation edge-cases.

docs/transient.org

@@ -522,6 +522,9 @@ Also see [[* Common Suffix Commands]].
   If you change the value of this option, then you might also
   want to change the value of ~transient-mode-line-format~.
 
+  This user option may be overridden if ~:display-action~ is passed
+  when creating a new prefix with ~transient-define-prefix~.
+
 *** Accessibility Options
 :PROPERTIES:
 :UNNUMBERED: notoc
@@ -560,6 +563,9 @@ Also see [[* Common Suffix Commands]].
   ~transient-key-exit~ (if allowed and they exit the transient) is
   used to draw the line.
 
+  This user option may be overridden if ~:mode-line-format~ is passed
+  when creating a new prefix with ~transient-define-prefix~.
+
   Otherwise this can be any mode-line format.  See [[info:elisp#Mode
   Line Format]], for details.
 
@@ -1951,6 +1957,12 @@ functions use ~describe-function~.
   against a specific value, as returned by ~transient-infix-value~,
   for example, ~--option=one~.
 
+- ~display-action~ determines how this prefix is displayed, overriding
+  ~transient-display-buffer-action~.  It should have the same type.
+
+- ~mode-line-format~ is this prefix's mode line format, overriding
+  ~transient-mode-line-format~.  It should have the same type.
+
 - ~scope~ For some transients it might be necessary to have a sort of
   secondary value, called a “scope”.  See ~transient-define-prefix~.
 
@@ -2170,7 +2182,9 @@ See [[*Enabling and Disabling Suffixes]].
 :UNNUMBERED: notoc
 :END:
 
-Yes, see ~transient-display-buffer-action~ in [[*Configuration]].
+Yes, see ~transient-display-buffer-action~ in [[*Configuration]].  You can
+also control how the popup buffer is displayed on a case-by-case basis
+by passing ~:display-action~ to ~transient-define-prefix~.
 
 ** How can I copy text from the popup buffer?
 :PROPERTIES:

docs/transient.texi

@@ -684,6 +684,9 @@ then that unfortunately changes which buffer is current.
 
 If you change the value of this option, then you might also
 want to change the value of @code{transient-mode-line-format}.
+
+This user option may be overridden if @code{:display-action} is passed
+when creating a new prefix with @code{transient-define-prefix}.
 @end defopt
 
 @anchor{Accessibility Options}
@@ -719,6 +722,9 @@ color of @code{transient-key-noop} (if non-suffixes are disallowed),
 @code{transient-key-exit} (if allowed and they exit the transient) is
 used to draw the line.
 
+This user option may be overridden if @code{:mode-line-format} is passed
+when creating a new prefix with @code{transient-define-prefix}.
+
 Otherwise this can be any mode-line format.  See @ref{Mode Line Format,,,elisp,}, for details.
 @end defopt
 
@@ -2214,6 +2220,14 @@ options and @code{transient-switches} suffixes it is also possible to match
 against a specific value, as returned by @code{transient-infix-value},
 for example, @code{--option=one}.
 
+@item
+@code{display-action} determines how this prefix is displayed, overriding
+@code{transient-display-buffer-action}.  It should have the same type.
+
+@item
+@code{mode-line-format} is this prefix's mode line format, overriding
+@code{transient-mode-line-format}.  It should have the same type.
+
 @item
 @code{scope} For some transients it might be necessary to have a sort of
 secondary value, called a ``scope''.  See @code{transient-define-prefix}.
@@ -2489,7 +2503,9 @@ See @ref{Enabling and Disabling Suffixes}.
 @anchor{Can I control how the popup buffer is displayed?}
 @appendixsec Can I control how the popup buffer is displayed?
 
-Yes, see @code{transient-display-buffer-action} in @ref{Configuration}.
+Yes, see @code{transient-display-buffer-action} in @ref{Configuration}.  You can
+also control how the popup buffer is displayed on a case-by-case basis
+by passing @code{:display-action} to @code{transient-define-prefix}.
 
 @anchor{How can I copy text from the popup buffer?}
 @appendixsec How can I copy text from the popup buffer?

lisp/transient.el

@@ -723,6 +723,8 @@ If `transient-save-history' is nil, then do nothing."
    (environment          :initarg :environment          :initform nil)
    (incompatible         :initarg :incompatible         :initform nil)
    (suffix-description   :initarg :suffix-description)
+   (display-action       :initarg :display-action       :initform nil)
+   (mode-line-format     :initarg :mode-line-format)
    (variable-pitch       :initarg :variable-pitch       :initform nil)
    (column-widths        :initarg :column-widths        :initform nil)
    (unwind-suffix        :documentation "Internal use." :initform nil))
@@ -3895,10 +3897,8 @@ have a history of their own.")
         (setq tab-line-format nil))
       (setq header-line-format nil)
       (setq mode-line-format
-            (if (or (natnump transient-mode-line-format)
-                    (eq transient-mode-line-format 'line))
-                nil
-              transient-mode-line-format))
+            (let ((format (transient--mode-line-format)))
+              (if (or (natnump format) (eq format 'line)) nil format)))
       (setq mode-line-buffer-identification
             (symbol-name (oref transient--prefix command)))
       (if transient-enable-popup-navigation
@@ -3913,12 +3913,9 @@ have a history of their own.")
       (when-let ((line (transient--separator-line)))
         (insert line)))
     (unless (window-live-p transient--window)
-      (when (eq (car transient-display-buffer-action)
-                'display-buffer-full-frame)
-        (user-error "Invalid value for `transient-display-buffer-action'"))
       (setq transient--window
             (display-buffer transient--buffer
-                            transient-display-buffer-action)))
+                            (transient--display-action))))
     (when (window-live-p transient--window)
       (with-selected-window transient--window
         (set-window-parameter nil 'prev--no-other-window
@@ -3929,6 +3926,13 @@ have a history of their own.")
           (transient--goto-button focus))
         (transient--fit-window-to-buffer transient--window)))))
 
+(defun transient--display-action ()
+  (let ((action (or (oref transient--prefix display-action)
+                    transient-display-buffer-action)))
+    (when (eq (car action) 'display-buffer-full-frame)
+      (user-error "Invalid value for `transient-display-buffer-action'"))
+    action))
+
 (defun transient--fit-window-to-buffer (window)
   (let ((window-resize-pixelwise t)
         (window-size-fixed nil))
@@ -3942,11 +3946,16 @@ have a history of their own.")
                               (window-body-width window t)
                               (window-body-height window t))))
 
+(defun transient--mode-line-format ()
+  (if (slot-boundp transient--prefix 'mode-line-format)
+      (oref transient--prefix mode-line-format)
+    transient-mode-line-format))
+
 (defun transient--separator-line ()
-  (and-let* ((height (cond ((not window-system) nil)
-                           ((natnump transient-mode-line-format)
-                            transient-mode-line-format)
-                           ((eq transient-mode-line-format 'line) 1)))
+  (and-let* ((format (transient--mode-line-format))
+             (height (cond ((not window-system) nil)
+                           ((natnump format) format)
+                           ((eq format 'line) 1)))
              (face `(,@(and (>= emacs-major-version 27) '(:extend t))
                      :background
                      ,(or (face-foreground (transient--key-face nil 'non-suffix)