mouse/keyboard focus with vnc fullscreen on one of two screens

[gregrwm]

When one screen is fullscreen in tigervnc-viewer, and i move the mouse onto the other screen, keyboard focus seems usually to remain in vnc, tho sometimes follows the mouse to the other screen. it's odd that it's inconsistent. And an annoyance to need to click, or dismiss fullscreen from vnc.

in my ongoing window manager hopping i note that it's sadly common among them to find that focus won't follow the mouse with vnc in fullscreen, and refreshing to find the ones where it will.

ubuntu 20.04 i3-wm:amd64 (4.17.1-1)
complete current i3config: i3config.txt