The extra space in the second frame's minibuffer remains for the life of
the frame AFAICT, at least I couldn't find any way to eliminate it.

This is on GNU Emacs 25.0.92.6 (x86_64-suse-linux-gnu, GTK+ Version
3.14.15) of 2016-03-26, emacs-repository-version
bc70fda2c9f93a30351c7c79a2b5763bbbd7bbc6, system-configuration-options
"--with-xwidgets 'CFLAGS=-Og -g3'", system-configuration-features XPM
JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND DBUS GCONF GSETTINGS NOTIFY
GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS
GTK3 X11 XWIDGETS

Steve Berman