and creating EmacsFSWindow instead of EmacsWindow (so canBecomeKeyWindow and canBecomeMainWindow both return YES).
But it didn't make the trick.
Let me know if you need any additional information.
In GNU Emacs 26.1 (build 1, x86_64-apple-darwin14.5.0, NS appkit-1348.17 Version 10.10.5 (Build 14F2511)) of 2018-05-31 built on builder10-10.porkrind.org Windowing system distributor 'Apple', version 10.3.1561 Recent messages: For information about GNU Emacs and the GNU system, type C-h C-a. user-error: Beginning of history; no preceding item user-error: End of history; no default available
Important settings: value of $LANG: en_GB.UTF-8 locale-coding-system: utf-8-unix
Major mode: Lisp Interaction
Minor modes in effect: tooltip-mode: t global-eldoc-mode: t eldoc-mode: t electric-indent-mode: t mouse-wheel-mode: t tool-bar-mode: t menu-bar-mode: t file-name-shadow-mode: t global-font-lock-mode: t font-lock-mode: t blink-cursor-mode: t auto-composition-mode: t auto-encryption-mode: t auto-compression-mode: t line-number-mode: t transient-mark-mode: t