Multiple files to <playground-ide> displays only 1st tab

## Issue
When multiple files are supplied to ``, only the first tab can be seen.

## Expected result
You can see the second tab in this example.

## Details
I'm loading playground via skypack CDN...
```

```
It looks like `` wraps each ``; I can see 2 tabs in my DOM, with the correct data bound to the elements, it looks like `` isn't setting a `display: flex` property correctly however, other styles aren't being set too, like underlining selected tab.

You can see the 2nd tab is in the DOM but overlapped by the content below it...

Here inline style for `display: flex` was manually added to `` for proof of concept; the orange arrow points to where the style has failed to inherit (not sure why)...

In an attempt reverse engineer, I mimicked the demo: https://polymerlabs.github.io/playground-elements/

...and then loaded the same script that was on the demo page (with code below), it fixes the issue but that source probably isn't a reliable URL, imagine the CDN is best.
```

```

Let me know if any other info is needed, thank you for your help :)