Jquery DIalog close button displaying with long border extending to side of the containing element

Jquery DIalog close button displaying with long border extending to side of the containing element

Firefox 89.0.2, OS Ubuntu 20.04, Jquery-ui v1.12.1 jquery v3.5.1

Steps to reproduce visit https://jqueryui.com/dialog/ on firefox. The dialog is displayed with an orange border extending to the left of the containing element. The same effect happen on Windows Firefox VM (though with a blue line) and has been experienced by other friends on different machines. Tabbing control will remove the line, but if you return focus to the close button it appears. I have not been able to find the issue in the CSS. Any ideas very welcome.