Scroll elements to middle of viewport, not to top

[stefanv]

(Elements can get hidden underneath a top bar, e.g.)

[pep8speaks]

Hello @stefanv! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file app/test_util.py:

Line 105:80: E501 line too long (108 > 79 characters)