Use case for new Progress widget

[Bryan]

Please see the discussion and resolution linked below, your issue is precisely the same:

Temporarily setting a line to not visible

The synchronization between Python and JS only happens at function exits, so the first line.visible = False has no effect since you change the value back to True before the function ends. You will need to split up your callback into two pieces using add_next_tick_callback to do the second part of the work in a separate function invocation. This technique is demonstrated here: https://docs.bokeh.org/en/latest/docs/user_guide/server/app.html#updating-from-threads