From 4f56ca63761b0270edb58734ce7ac4a3c800bb1b Mon Sep 17 00:00:00 2001
From: Po Lu <luangruo@yahoo.com>
Date: Fri, 3 Jun 2022 15:20:23 +0800
Subject: [PATCH] Add new user option `mouse-drag-mode-line-buffer'

* etc/NEWS: Announce new option.  Also add missing entries for
an earlier change.
* lisp/mouse.el (mouse-drag-mode-line-buffer): New user option.
(mouse-drag-mode-line): Implement that option.
---
 etc/NEWS      | 10 +++++++---
 lisp/mouse.el | 30 +++++++++++++++++++++++++++++-
 2 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index 3870e937df8..f9409bb24f8 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -305,6 +305,10 @@ the current Emacs process at the end, it starts a new Emacs process
 optional parameters to restart instead of just killing the current
 process.
 
+** New user option 'mouse-drag-mode-line-buffer'.
+If non-nil, dragging on the buffer name part of the mode-line will
+drag the buffer's associated file to other programs.
+
 +++
 ** New user option 'mouse-drag-and-drop-region-cross-program'.
 If non-nil, this option allows dragging text in the region from Emacs
@@ -2083,9 +2087,9 @@ list in reported motion events if there is no frame underneath the
 mouse pointer.
 
 +++
-** New function 'x-begin-drag'.
-This function initiates a drag-and-drop request with the contents of
-the selection 'XdndSelection', and returns when a drop occurs.
+** New functions 'x-begin-drag', 'dnd-begin-text-drag' and 'dnd-begin-file-drag'.
+These functions allow dragging contents (such as files and text) from
+Emacs to other programs.
 
 ---
 ** New function 'ietf-drums-parse-date-string'.
diff --git a/lisp/mouse.el b/lisp/mouse.el
index 5834d4a666e..1f2e9252228 100644
--- a/lisp/mouse.el
+++ b/lisp/mouse.el
@@ -108,6 +108,13 @@ down and up respectively."
                  (integer :tag "This many lines from window top or bottom"))
   :version "29.1")
 
+(defcustom mouse-drag-mode-line-buffer nil
+  "If non-nil, allow dragging files from the mode line.
+When the buffer has an associated file, it can be dragged from
+the buffer name portion of its mode line to other programs."
+  :type 'boolean
+  :version "29.1")
+
 (defvar mouse--last-down nil)
 
 (defun mouse--down-1-maybe-follows-link (&optional _prompt)
@@ -854,8 +861,29 @@ frame instead."
   (interactive "e")
   (let* ((start (event-start start-event))
 	 (window (posn-window start))
-         (frame (window-frame window)))
+         (frame (window-frame window))
+         (skip-tracking nil)
+         filename)
+    ;; FIXME: is there a better way of determining if the event
+    ;; started on a buffer name?
+    (when (and mouse-drag-mode-line-buffer
+               (eq (car (posn-string start))
+                   (car (with-selected-window window
+                          (setq filename (buffer-file-name))
+                          mode-line-buffer-identification)))
+               filename
+               (file-exists-p filename))
+      (let ((mouse-fine-grained-tracking nil))
+        (track-mouse
+          (setq track-mouse 'drag-source)
+          (let ((event (read-event)))
+            (if (not (eq (event-basic-type event)
+                         'mouse-movement))
+                (push event unread-command-events)
+              (dnd-begin-file-drag filename frame 'copy t)
+              (setq skip-tracking t))))))
     (cond
+     (skip-tracking t)
      ((not (window-live-p window)))
      ((or (not (window-at-side-p window 'bottom))
           ;; Allow resizing the minibuffer window if it's on the
-- 
2.39.2