Fix position for dialogs. (mathjax/MathJax#3480)

[dpvc]

This PR fixes a problem with dialogs opening at the top of the page and sometimes scrolling to the top. The original position: relative CSS is the problem, and it is resolves by using position: fixed. That means the dialogs don't scroll with he page as they did in 4.1, but I don't see a problem with that.

Resolves issue mathjax/MathJax#3480.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.50%. Comparing base (82cabf4) to head (d1ff109).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1403   +/-   ##
========================================
  Coverage    86.50%   86.50%           
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  Files          340      340           
  Lines        85952    85952           
  Branches      4816     4816           
========================================
  Hits         74357    74357           
  Misses       11572    11572           
  Partials        23       23           

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