Preserve existing cell output as a permanent exemplar
If you use notebooks as reproducible documentation, sometimes you may want to replay a notebook interactively, but want to keep the existing output as a reference.
This extension allows you to take existing output from a cell and save it as an "Exemplar output", which gets attached to an ExemplifiedOutput entry in the metadata.
When a cell has existing ExemplifiedOutput, it gets rendered to the side of the cell's latest output:
The Exemplar output expands on mouse over to full width.
Internals
This extension's logic largely takes from the Execute Time extension