>  > I wonder why it doesn't work to drop the text onto other applications.
>
>  Probably because there's no standard protocol to drag and drop anything
>  ​​but files between different applications.
>
> ​​Maybe not for every window system but on MacOS there is and many applications accept dragged text
> directly:
> ​​https://developer.apple.com/documentation/uikit/drag_and_drop/making_a_view_into_a_drag_source
> ​ - source info​

For now `mouse-drag-and-drop-region' does nothing when
(event-end (read-event)) is toward out of the current frame.

I think the first step is to revise `mouse-drag-and-drop-region' to let
it drag text to a different frame.