Progress bar does not provide enough visual information during long page loads

[TheWitness]

Describe the bug

When a page load takes a long time, say for example when a long running callback is being executed, the top progress bard will go all the way to the right, and then it becomes hard to distinguish when the page has actually finished. Instead, place a progress bar in the center of the page to provide that progress indicator.

Expected behavior

Make it more clear that the callback is still working.