When zooming or panning, if we have tiles at alternate resolutions, those tiles are inserted in the DOM before the tiles at the target resolution. This means you should see blurry tiles instead of whitespace when panning (assuming you've already zoomed in a bit).
Tile pruning code separated into to methods for clarity. Alt-z tiles are only removed when all current-z tiles have loaded. Thanks @elemoine for the excellent debugging session.
Tim Schaub
ahocevar
Tom Payne