https://github.com/ngtk3/NEd

Gdk-CRITICAL **: gdk_window_move_resize_internal: assertion 'GDK_IS_WINDOW (window)' failed

I get that message for GTK 3.18 when I launch the editor from the
command line with two file arguments, and then use HSplit or VSplit
from gears menu. But only when automatic scrollbars with overlay are
selected in preferences. That is the default.

No idea. I tried some hours to modify my code, but no change. Is that a
known bug of GTK 3.18? Well, gedit seems to have it not, so there may
be a way to suppress it.