The build history sidepanel widget calls Run#getTruncatedDescription to render a subset of a build's description inside itself: https://github.com/jenkinsci/jenkins/blob/d2a4a44a5c10ff9f32632fdd628deed275ea39a1/core/src/main/resources/hudson/widgets/HistoryWidget/entry.jelly#L81

This method is bad as explained in its Javadoc: https://github.com/jenkinsci/jenkins/blob/d2a4a44a5c10ff9f32632fdd628deed275ea39a1/core/src/main/java/hudson/model/Run.java#L696

A different, better approach would be to do the following:

1. Have the configured MarkupFormatter render the entire description of the build as HTML.
2. Truncate that (using similar rules)
3. Pipe the result through an HTML formatter to ensure valid HTML (e.g., all tags that were opened are closed)
4. Use that.