Upon upgrading from 6.1 (I think) to 6.3, there appears to be no style for .tabs-panel.is-active to switch the panel from display none to block. This effectively makes tabs useless.
Here are the styles applied by the old version to my active panel:
And here's 6.3:
In addition, a text search of the css shows that style no longer exists, and there are no indications of changes in the docs.
Can someone help?