mimic i3 ability to fullscreen parent

[adlainewson]

in i3 I can use the following to get a 'maximized' container without going fullscreen

bindsym button3 focus parent; fullscreen; focus child

in sway the result is simply fullscreen. The i3 behaviour is very helpful for controlling the desktop only with mouse, as returning from fullscreen otherwise requires keyboard input (while in i3 the operation is reversible with the mouse)

EDIT: clarification: sway behaves like i3 when using sequential keypresses to focus parent then fullscreen, and only differs from i3 when using the above mouse binding