FAILED assert(object_no < m_object_map.size())
Steps to reproduce:
1. clone a large image
2. create a snapshot on the clone
3. shrink the clone twice
The semi-recent addition of deep-flatten incorrectly computes the copy-up region using the largest parent overlap. Since the first shrink would have handled the copy-up for the trimmed region, always use the most recent parent overlap. Using the snapshot's overlap will result in an attempt to go beyond the current object map.