* Fix some compatibility issues with 3rd party themes.
* Keep urlbar suggestion dropdown arrow visible.
* Add option to hide background/async/XHR network status.
* Implement a simple style editor for selecting progress colors and images.
* Fix toolbar progress settings.
* Add annotations to the status elements. #status4evar-status-text, #statusbar-display, and .urlbar-over-link-box will have a "type" attribute set to one of the following: link, network, network_xhr, status_chrome, status_content, status_content_default, or status_chrome_default.
So my guess is that it's a setting to toggle showing the network status for operations that don't cause a page refresh. I haven't tested it yet though.
On a side note, since some of the options may look quite cryptic to the average non-tech user, it would be nice to have tooltips with explanations in the Options pane.