The `className` prop for `UnderlinePanels` is not passed along to `tab-container`

[joshblack]

The className prop for UnderlinePanels is not passed along to the underlying tab-container custom element. Instead, it is provided to the tablist wrapper. This may or not be an issue, but I wanted to make the bug report because tab-container exposes elements in its Shadow DOM through part. If the className is not provided to the outermost tab-container, then folks cannot. use ::part() to style the different parts exposed by the shadow DOM of tab-container

[hellojanehere]

@joshblack what's the status of this? I see you opened a PR against it last month?

[joshblack]

@hellojanehere my memory of this is that this would be a fix that could break usage downstream (see also: https://github.com/primer/react/pull/6292#issuecomment-3059330671)

Next steps I think would be to have someone test this out to see if any breakage occurs and then determine next best steps (either address breakage if it exists or merge as-is).