This fixes block headers overlapping over text in the buffer when using
a custom line height of 1.25.

It fixes the issue by making the parent container a v-flex,
vertically-justifying the content and moving from relative padding to
absolute padding for the header itself.

Co-authored-by: antonio <antonio@zed.dev>
Co-authored-by: julia <julia@zed.dev>
Co-authored-by: marshall <marshall@zed.dev>