FocusBuilder

• widget()

  The function widget() adds widgets for the focus. They will be traversed in the order given.

• start() and end() are used to define containers.

The two other important functions are

• build_focus()

  Takes a container widget and returns a Focus.

• rebuild_focus()

  Does the same, but takes the previous Focus too.

  What it does is, it builds the new Focus and checks which widgets are no longer part of it. It resets all FocusFlags for those widgets.

  A bonus is it reuses the allocations too.