Fix layout of login page bloom

What does this MR do and why?

Prevent the login page's bloom backdrop from scrolling with the body if the viewport is very short.

References

(reported in internal slack thread)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Check out this branch
2. Log out in the GDK
3. make the viewport very short so that the login screen has vertical scroll
4. Scroll up and down; the bloom background will not scroll.

MR acceptance checklist

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