Comment 1 for bug 1669437

This is mentioned in item one of the release notes:
https://peppermintos.com/release-notes/

We're looking into fixing the "Action Buttons" xfce4-panel plugin for Peppermint 8 and if/when fixed, we'll probably backport the fix to Peppermint 7.

I'm sorry to say the "Action Buttons" plugin is one of the few Xfce panel plugins that don't currently work properly with our mix of the xfce4-panel and LXDE session management .. but whilst you wait for Peppermint 8 or a backported fix, you might find this useful:
https://forum.peppermintos.com/index.php/topic,3728.0.html