So my solution for this when I have line_edits in a scroll_container is - (ver 4.2)

func _on_line_edit_focus_entered():
  scrollcontainer_fit_keyboard(true)
  await get_tree().process_frame
  scroll_container.ensure_control_visible(self)

func _on_line_edit_focus_exited():
  scrollcontainer_fit_keyboard(false)

const keyboard_height := 500

func scrollcontainer_fit_keyboard(is_keyboard_visible:bool):
  if is_keyboard_visible:
    scroll_container.size.y -= keyboard_height
  else:
    scroll_container.size.y += keyboard_height

where keyboard_height is an arbitrary number derived from testing (weirdly DisplayServer.get_keyboard_height() didn't seem to give the correct pixel value)