> Date: Mon, 13 Apr 2020 10:21:50 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: emacs-devel@gnu.org
> 
> So perhaps instead of using w32_system_caret_hwnd, you should use the
> return value of GetFocus function, similar to what we do elsewhere in
> w32_msg_pump.

And if GetFocus returns NULL, skip the processing, like you do if
get_ime_context_fn returns NULL.