The following screenshot shows 2 issues with the login box top left:
1.The 2 text fields are too large.
2. The buttons for third-party authentication, such as Google's, which appear under "Sign In using", are nearly completely hidden.

The second issue occurs with my Mozilla Firefox 52.0.9 on Debian stable, at default zoom level. It does not happen with 2 other Firefox installs on Microsoft Windows.