A new option for configuration variable value for MS_Thumbnail:

MS_Thumbnail = BOUNDARY

By setting this configuration variable with this new value, the Sheet Model Boundary area will be used as the DGN thumbnail.

The image captured for the DGN thumbnail will be everything inside sheet boundary.

Where there are multiple Sheet Models, then the last Sheet Model edited will be displayed as the thumbnail.

An optional variation on this new variable value can be for DGNs with multiple Sheet Models could be: 'MS_Thumbnail = BOUNDARY_FIRST' which is the first Sheet Model Boundary index/order in the file.