There is a user option, dnd-open-file-other-window, which should
control the behaviour of drag-n-dropping files onto an Emacs frame.
I customized it to t:
dnd-open-file-other-window is a variable defined in `dnd'.
Its value is t
Documentation:
If non-nil, always use find-file-other-window to open dropped files.
However, that had no effect. :-(
Meanwhile, the Emacs manual is also quite funny. It states in section
25.15, Drag and Drop:
Dropping a file normally visits it in the window you drop it on. If
you prefer to visit the file in a new window in such cases, customize
the variable `x-dnd-open-file-other-window'.
But the variable `x-dnd-open-file-other-window' does not exist. :-(
I noticed this misbehaviour in Emacs version