* lisp/outline.el: Fix the value 'insert' of outline-minor-mode-use-buttons.

(outline--insert-button): Keep text properties around point
on the inserted whitespace placeholder.

* lisp/apropos.el (apropos-mode):
* lisp/progmodes/xref.el (xref--xref-buffer-mode):
Change outline-minor-mode-use-buttons from t to 'insert'.

lisp/apropos.el

@@ -496,7 +496,7 @@ Intended as a value for `revert-buffer-function'."
               outline-level (lambda () 1)
               outline-minor-mode-cycle t
               outline-minor-mode-highlight t
-              outline-minor-mode-use-buttons t))
+              outline-minor-mode-use-buttons 'insert))
 
 (defvar apropos-multi-type t
   "If non-nil, this apropos query concerns multiple types.

lisp/outline.el

@@ -1817,7 +1817,7 @@ With a prefix argument, show headings up to that LEVEL."
         (unless o
           (when (eq outline-minor-mode-use-buttons 'insert)
             (let ((inhibit-read-only t))
-              (insert "  ")
+              (insert (apply #'propertize "  " (text-properties-at (point))))
               (beginning-of-line)))
           (setq o (make-overlay (point) (1+ (point))))
           (overlay-put o 'outline-button t)

lisp/progmodes/xref.el

@@ -994,7 +994,7 @@ point."
   (setq-local add-log-current-defun-function
 	      #'xref--add-log-current-defun)
   (setq-local outline-minor-mode-cycle t
-              outline-minor-mode-use-buttons t
+              outline-minor-mode-use-buttons 'insert
               outline-search-function
               (lambda (&optional bound move backward looking-at)
                 (outline-search-text-property