mahkoh
No "focus child" command
There is no "focus child" command, which would have the opposite behavior of "focus parent".
When you have a horizontal split and the split is focused, you can focus the child that previously had the focus by pressing alt-j. Is that what you're looking for?
I've actually tried to find a way to create a shortcut to move focus from a container, but only just now realized that all I need is a chain of focus-parent, focus-right, at least for my use case. Oops, sorry.
But fwiw, in sway this command would change focus from the whole split back to the child that had focus when user issued the focus parent command
You can also kind of feel that this action is missing if you would focus parent so much, that you have to return focus with your mouse. I haven't found a way to return back from this situation otherwise.
