Fix that the progress bar would fill the entire screen in Safari 6

Perhaps some kind of stacking context (https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Understanding_z_index/The_stacking_context) issue.

Fixes #436
Closes #437
Merge pull request #440 from javan/fix-progress-bar-in-safari-6

Fix that the progress bar would fill the entire screen in Safari 6