ChromeOS  11021.21.0, 70.0.3538.25  dev-channel  cave

What steps will reproduce the problem?
1. On the sign-in screen, click on uber tray and connect to some captive portal network.
2. Dismiss the captive portal authentication window without entering password.
3. Click on 'Add Person' button.
4. 'Network not available' window is displayed. Observe 'Back' button on bottom left corner of the window

What happens?
Back button overlaps with "If you've already registered on this device..." text.