DataBrowser loading state transition improvement.

[subdavis]

background

Originally, DataBrowser would show a single row of "No data available" during load. I changed this intentionally because, when girder response times are quick, it caused the entire component (and potentially the page) to flicker in an undesirable way.

Now, during load, the old directory contents stick around until the new ones are ready. This doens't look right at slow speeds.

Changes requested:

• When navigating between a meta-directory (like users) where there are never check boxes and a real directory, don't show checkboxes until the new contents load.
• During load, disable check and click behavior.

[zachmullen]

During load, disable check and click behavior.

Ideally, this should be done in a visually obvious way so that the user realizes they're looking at the old directory contents and not the new one, since there will be a temporary mismatch with the breadcrumb.