See attached screenshot.

#timestamper-pane.top isn't enough anymore. 

https://github.com/jenkinsci/timestamper-plugin/blob/106332cde359ddbafa49dceb5f6381d6c5ad7c87/src/main/resources/hudson/plugins/timestamper/annotator/TimestampAnnotatorFactory3/usersettings.jelly#L34 

Something like 80 seems reasonable.