Gap after border in markdown preview from header

[nihaals]

Describe the bug

If a markdown element with top padding is at the start of the markdown file, a weird gap is added between the border and the start of the markdown content. Since a lot of READMEs have a header at the start, this is fairly common.

To Reproduce

1. Upload a package zip to PackageUpload with a README which starts with # Header
2. Notice the gap between the border and the header in the markdown preview

Expected behaviour

There is a regular gap between the border and the header (like with regular text instead of a header).

Screenshots

Gap with heading

Normal gap

Additional context

Add any other context about the problem here.
Browser: Chrome 91.0.4472.114, Linux
Version: 17a01d7