Sorry for the slightly contrived way these bugs are described.  I'm
working on them already for a couple of days and even smallest changes
to the scenario below makes them disappear.  To reproduce proceed as
follows:

(1) Save the attached foo.el file somewhere and make its first line match
     the location where you stored it.

(2) Start Emacs with the options -Q and -l to load foo.el.

(3) Type C-x 5 2.

(4) Go back to the initial frame, move to the end of the last non-empty
     line after ";; (bar)" and type C-x C-e.

At this moment "nothing" happens here (Bug#1).  When I now switch (via
Alt TAB) to the new frame (the one created via C-x 5 2), the message
appears there.  When I now type C-p in the new frame, the minibuffer
window shrinks back but the space previously occupied by the modeline of
the window above is not redrawn, hence I get two modelines above each
other (Bug#2).

Bug#1 can be observed here on the Gtk3, Lucid and Windows builds, Bug#2
only on Lucid and Windows.  Bugs appear for both, Emacs-25 and master.

Note that Bug#1 does not appear, for example, when I display a one line
message, when the new frame displays a different buffer, or when point
in the new frame is not a EOB.

I should be eventually able to track this down but if someone beats me
to it or has any ideas ...

martin