Smart Mouse Drags between Windows

If an active (highlighted) region exists within the editor:
  ACTION KEY
     Copies and yanks (pastes) the region to the release point in a
     different window.
  ASSIST KEY
     Kills (cuts) and yanks (pastes) the region to the release point
     in a different window.

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Otherwise, if dragged from inside one window to another:
  ACTION AND ASSIST KEYS
     (1) If depress was on a buffer name in Buffer-menu/ibuffer mode or on
         a file/directory in dired mode, displays the item in window of release.
         If the drag start position is within a button, displays the button
         referent in window of release.
         See hmouse-drag-item-mode-forms for how to allow for draggable
         items in other modes.
     (2) Otherwise, creates a new link button at the drag start location,
         linked to the drag end location.  Action Key creates an implicit button;
         Assist Key creates an explicit button.

         In Hyperbole versions prior to 9, Assist Key drags between windows would
         swap buffers.  In version 9 and above, start or end the drag between
         windows on a modeline to get this same behavior.

Smart Mouse Drags between Windows ​

Smart Mouse Drags between Windows