On Sat, Mar 18 2017, Eli Zaretskii wrote:
> Can you try disabling double buffering?  Like this:
>
>   (add-to-list 'default-frame-alist '(inhibit-double-buffering . t))

Ah! This affects the issue.

By showing the same buffer on two frames, the original frame shows the
problem, the new frame without double-buffering does not show any
artifact.

Screenshot attached. Old frame (with double-buffering) on the top.
This is the same emacs instance showing the same buffer.