Set popover boundary to viewport

[derdiggn]

Fixes #655

[derdiggn]

Hello @sebastianbergmann,
this is a small fix four the positioning problem.

regards 

[codecov]

Codecov Report

Merging #659 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #659   +/-   ##
=========================================
  Coverage     85.16%   85.16%           
  Complexity      805      805           
=========================================
  Files            32       32           
  Lines          2534     2534           
=========================================
  Hits           2158     2158           
  Misses          376      376

Impacted Files	Coverage Δ	Complexity Δ
src/Report/Html/Renderer/File.php	91.15% <100%> (ø)	110 <0> (ø)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cdba7cf...3ebe48b. Read the comment docs.

[sebastianbergmann]

Thank you for your contribution. I appreciate the time you invested in preparing this pull request. However, I have decided to merge #658 instead.