* `on.mouse_focus(in_focus)` -- called when the mouse pointer moves on or off
  the app window. `in_focus` will be `true` when mouse comes within the window
  area, and `false` when it goes off.
  (Based on [LÖVE](https://love2d.org/wiki/love.mousefocus).)

end
function App.mousefocus(in_focus)
  if Mode == 'error' then return end
  Cursor_time = 0  -- ensure cursor is visible immediately after it moves
  if on.mouse_focus then on.mouse_focus(in_focus) end