Fix mouse dragging of vertical dividers with scroll bars on left (Bug#23690)

* lisp/mouse.el (mouse-drag-line): With scroll bars on the left
adjust trailing edge of window on the left when dragging the
vertical divider of the mode line.  (Bug#23690)

1 files changed, 9 insertions, 1 deletions

diff --git a/lisp/mouse.el b/lisp/mouse.el
index fa355ffeb71..592338ad09d 100644
--- a/lisp/mouse.el
+++ b/lisp/mouse.el
@@ -406,7 +406,15 @@ must be one of the symbols `header', `mode', or `vertical'."
                            (or (not resize-mini-windows)
                                (eq minibuffer-window
                                    (active-minibuffer-window)))))))
-          (setq draggable nil))))
+          (setq draggable nil)))
+     ((eq line 'vertical)
+      (let ((divider-width (frame-right-divider-width frame)))
+        (when (and (or (not (numberp divider-width))
+                       (zerop divider-width))
+                   (eq (cdr (assq 'vertical-scroll-bars
+                                  (frame-parameters frame)))
+                       'left))
+        (setq window (window-in-direction 'left window t))))))
 
     (let* ((exitfun nil)
            (move