Dialogs are not scrollable in responsive mode

[basitanwer]

Head over to the Dialog example and open it on mobile.
Try Sign in or Dialog with image

[marlier]

This happens any time the Dialog is larger than the viewport.

This can be fixed with a theme override:

<ThemeProvider
                  value={{
                    dialog: {
                      styles: {
                        base: {
                          backdrop: {
                            position: 'relative',
                          },
                        },
                      },
                    },
                  }}
                >

I've opened PR #648 for this.