blue bounding box

What is this blue bounding box which has appeared on my bitmap layers? It resides outside of the storyboard panel and seems to function as some sort of mask? Any ideas? Thanks.

I have this problem too- The solution is to go to Storyboard ( top bar menu) - properties then change the bitmap resolution. . I do think SBpro should be able to figure out that the vector drawing you want to convert should not be clipped .Maybe it will come in newer versions .