From the help: Enlarge by synthesizing a concentric band that matches the edge, maintaining perspective. Works best for small enlargement with natural edges. Undo a Crop instead if possible! Requires separate resynthesizer plug-in.
This is the initial version, lightly tested.
Examples show that the results are not always acceptable: undesirable artifacts appear in undesirable regular patterns. In other words, hit and miss. The attached examples are the result of Uncrop at 20 percent.