Remove the border. Fix #48765

[shizengzhou]

@bpasero I remove the border to fix it.

[bpasero]

@shizengzhou not a good change, this border is needed when the tab.border color is set, see:

[bpasero]

Maybe we should conditionally set this border if tab.border is defined, I think that should be possible 👍

[msftclas]

All CLA requirements met.

[shizengzhou]

@bpasero I have changed my code.

[bpasero]

@shizengzhou I did not realize that we set a border color by default in dark and light theme and we cannot just remove it because the color is actually in use:

As you can see, there is a slight black line between the two tabs. I think a better approach would be to not use borders at all for this kind of separation and have it some other way...

[shizengzhou]

@bpasero Now I don't use borders for the separations.

[bpasero]

@shizengzhou how about keeping the border as is and letting the special case of having a border for the active tab use the ::pseudo element trick? I actually do not like how it currently uses box-shadow so then we could get rid of that hack too.

[bpasero]

I found a simpler way via fd7d510

Thanks for the work here, sorry for not pushing it 👍