We believe we have discovered a bug in GridCoverageRenderer: when the layer is panned entirely off screen, a NullPointerException is thrown. Further panning of the map fails, always returning a blank (white) screen. Env is win10/java1.8/gt15-rc1 (also noted in gt14.1)
The issue is reproducable using the attached maven project. Click 'pan right' and 'pan left' to see that both buttons work as anticipated. Now click 'pan right' until the gridcoverage is entirely offscreen. You will see a NullPointerException thrown. Now click 'pan left' and you will see that whilst the map image appears to be redrawing, the result is a white screen.
I also attach a proposed fix for this issue GridCoverageRenderer.patch
Have also sent to devel list, but that has already bounced once due to not liking attachments, so am logging here to make sure it gets through