I found bug on `mouse-drag-and-drop-region'.  The
`mouse-drag-and-drop-region' does not recognize the beginning and the
end of highlighted text as pre existing region.

1: emacs -Q
2: (setq mouse-drag-and-drop-region 'meta)
2: Select text on a buffer and have a region. Let's assume `This buffer' is highlighted.
3: Drag and drop the text onto the first character of the region, that is `T'.
4: Then the text `This buffer' would be disappeared.

I attach a patch to fix the bug.  This patch is relative to 26.1.50.

* Commit log

Author: Tak Kunihiro <tkk@misasa.okayama-u.ac.jp>

    Fix bug of 'mouse-drag-and-drop-region' to detect edges of region
    
    * lisp/mouse.el
    (mouse-drag-and-drop-region): Detect both the beginning and the
    end of character of region during dragging text.