Mosaic background behaviour is odd.
The first two attachments, mosaicBackground and mosaicTransparent, show what happens with an irregularly shaped mosaic when moving around an "inner corner of it":
- mosaicBackGround has on the top two tiles that keep the black color of the background everywhere behind them
- panning a little down, so that the two tiles are out of the bounds, makes the same area become white
The third image has been generated using the same irregularly shaped mosaic but using OpenLayers in tiled mode, the result is that depending on the tile you might get the black background, a white background, or the "missing image" filler.
Simone, if you want I can try to provide a patch.