Pull Request comment entry form does not appear in Safari Version 11.1.1 (13605.2.8)

1. Open a Pull Request in Safari Version 11.1.1 (13605.2.8)
2. Hover over the gutter to make the + icon appear
3. Click the + icon

I should see a comment entry form, but I only see a thin gray bar. The form itself is not rendered.

This issue also occurs in the Safari Technology Preview Release 58 (Safari 12.0, WebKit 13606.1.18.2)