Yes, the problem goes away after reverting this commit.

Thanks Eli

Le jeu. 31 oct. 2019 à 00:18, Eli Zaretskii <eliz@gnu.org> a écrit :
severity 37097 normal
merge 37986 37097
thanks

> From: Sébastien Chapuis <sebastien@chapu.is>
> Date: Wed, 30 Oct 2019 17:00:00 +0800
>
> $ git clone --recursive git://git.savannah.gnu.org/emacs.git
> $ cd emacs && ./autogen.sh
> $ mkdir build && cd build
> $ ../configure --prefix=/opt/emacs-git --with-modules --with-xwidgets --with-imagemagick
> $ make install
> $ /opt/emacs-git/bin/emacs --fg-daemon -Q
>
> Open another terminal (terminal #2):
> $ /opt/emacs-git/bin/emacsclient -n -c BIG_FILE.txt
> the command doesn't return and no frame is opened.
> On the terminal #1, there is this output:
>
> ```
> Warning: due to a long standing Gtk+ bug
> https://gitlab.gnome.org/GNOME/gtk/issues/221
> Emacs might crash when run in daemon mode and the X11 connection is unexpectedly lost.
> Using an Emacs configured with --with-x-toolkit=lucid does not have this problem.
> Starting Emacs daemon.
> File BIG_FILE.txt is large (11.9 MiB), really open? (y)es or (n)o or (l)iterally
> File BIG_FILE.txt is large (11.9 MiB), really open? (y)es or (n)o or (l)iterally
> ```
>
> Open a 3rd terminal (terminal #3):
> $ /opt/emacs-git/bin/emacsclient -n -c
> A new frame is opened with this message in the minibuffer:
> `File BIG_FILE.txt is large (11.9 MiB), really open? (y)es or (n)o or (l)iterally`
> If I type `y`, a new frame is opened (from terminal #2) with the file BIG_FILE.txt in its buffer.

Thanks.  Can you see if the problem goes away if you revert commit
49fc040?