Gtk.Widget.prototype.child_focus

function child_focus(direction:Gtk.DirectionType):Boolean {
    // Gjs wrapper for gtk_widget_child_focus()
}

This function is used by custom widget implementations; if you're writing an app, you'd use Gtk.grab_focus to move the focus to a particular widget, and Gtk.Container.prototype.set_focus_chain to change the focus tab order. So you may want to investigate those functions instead.

Gtk.Widget.prototype.child_focus is called by containers as the user moves around the window using keyboard shortcuts. direction indicates what kind of motion is taking place (up, down, left, right, tab forward, tab backward). Gtk.Widget.prototype.child_focus emits the Gtk.focus signal; widgets override the default handler for this signal in order to implement appropriate focus behavior.

The default ::focus handler for a widget should return true if moving in direction left the focus on a focusable location inside that widget, and false if moving in direction moved the focus outside the widget. If returning true, widgets normally call Gtk.grab_focus to place the focus accordingly; if returning false, they don't modify the current focus location.