Thank you for the comments.  I revised English as suggested.

>> +(defun mouse-on-region-p (position &optional start end)
>> +  "Return if POSITION is in between START and END in the current buffer.
>> +When START and END are nil but there is active region, those of
>> +active region is fed."
>> +  (when (region-active-p)
>> +    (setq start (or start (region-beginning)))
>> +    (setq end (or end (region-end))))
>> +  (let ((point (posn-point position)))
>> +    (and
>> +     (numberp start)
>> +     (numberp end)
>> +     (numberp point)
>> +     (<= start point)
>> +     (<= point end))))
>
> This algorithm will fail with bidirectional text, where buffer
> positions don't increase monotonically with screen coordinates.  How
> about testing the face of the character under mouse instead?

I revised the code using following statements.  Accordingly,
`mouse-on-region-p' is not necessary anymore.

(equal (mouse-posn-property (event-start start-event) 'face) 'region)
(equal (mouse-posn-property (event-start start-event) 'face) 'secondary-selecton)

>> +(defun mouse-drag-region-pasting (event)
>
> The function's name is confusing.  Why not name it like the defcustom?

I changed the name from `mouse-drag-region-pasting' to
`mouse-drag-and-drop-region'.

> Please add a NEWS entry and some minimal documentation in the user
> manual.

I add ChangeLog, NEWS, and Info as shown below.  I think that all
concerns were resolved for now.


# ChangeLog

2017-05-16  Tak Kunihiro  <tkk@misasa.okayama-u.ac.jp>

	Support drag and drop region by the mouse (Bug#26725)

	* doc/emacs/frames.texi (Drag and Drop): Document support of drag
    and drop region by the mouse.
	* lisp/mouse.el (mouse-drag-region): Call mouse-drag-and-drop-region
    when start-event is against region.
    (mouse-drag-and-drop-region): Move the region by dragging the mouse.

# NEWS

** Support drag and drop text by the mouse.
You can start drag-and-drop text by customizing 'mouse-drag-and-drop-region'.

# Info

diff --git a/site-lisp/frames.252.texi b/site-lisp/frames.texi
index 1611bd1..da140a6 100755
--- a/site-lisp/frames.252.texi
+++ b/site-lisp/frames.texi
@@ -1046,12 +1046,17 @@ Window Dividers, elisp, The Emacs Lisp Reference Manual}.
 @cindex drag and drop

   In most graphical desktop environments, Emacs has basic support for
-@dfn{drag and drop} operations.  For instance, dropping text onto an
-Emacs frame inserts the text where it is dropped.  Dropping a file
-onto an Emacs frame visits that file.  As a special case, dropping the
-file on a Dired buffer moves or copies the file (according to the
-conventions of the application it came from) into the directory
-displayed in that buffer.
+@dfn{drag and drop} operations.  For instance, dragging region can move
+the entire region of text to point where mouse is dragged over to.
+Dropping text onto an Emacs frame inserts the text where it is dropped.
+Dropping a file onto an Emacs frame visits that file.  As a special
+case, dropping the file on a Dired buffer moves or copies the file
+(according to the conventions of the application it came from) into the
+directory displayed in that buffer.
+
+@vindex mouse-drag-and-drop-region
+  If you prefer to move the entire region of text by mouse, customize
+the variable @code{mouse-drag-and-drop-region}.

 @vindex dnd-open-file-other-window
   Dropping a file normally visits it in the window you drop it on.  If