Odd CSS behavior - can't find the controlling line anywhere

Notice how the tab border does not enclose the content? I have looked at the enclosing CSS and can’t seem to find where the issue is coming from.

I noticed this behaviour after I upgraded BF from version 8.111 to 9.x. I was told it is a bug . Interestingly, it is working perfectly in small screen like mobile. On desktop, if you resize your screen , it display correctly.