I was looking for this too, and came up with a solution I thought might be worth sharing. I haven’t tested it across too many browsers yet, but it’s working for me. Uses just one bit of css in the space-style to hide the frontmatter block down to just the first line (showing that there is frontmatter) until you move the cursor into it to make edits.
.sb-frontmatter.sb-line-frontmatter-outside:has(+ .sb-frontmatter) ~ .sb-frontmatter {
display:none
}