Flash of old prompt on enter

[jcubic]

Issue summary

When you have a dynamic prompt (using variable) and command change that prompt, there is a flash of old prompt before the new is visible.

Expected behavior

The new prompt should render without flash

Actual behavior

There is flash of old prompt after enter

Steps to reproduce

1. Open demo https://codepen.io/jcubic/pen/KKYBWog?editors=0010
2. type cd ~/directory
3. type cd /

Additional notes

This is related to #923

As a workaround, you can make the function that change the prompt async.

[jcubic]

Fixed with #923

[jcubic]

This is not fixed.