fix: make colorbox content scrollable

[saiqulhaq]

I am trying to fix this issue https://github.com/simplecov-ruby/simplecov-html/issues/58

This is a generated coverage file coverage.zip based on my gem, please open the index.html file https://www.loom.com/share/b263c41540f348f899b191953a13a2c5

I would like to refactor this PR and improve the performance by throttling the scroll event but I need to receive any feedback from maintainers first

Thanks