Change the default face for cell separators

[memeplex]

This default:

code-cells-header-line '((t :extend t :inherit header-line))

is rather problematic:

1. It makes difficult to tell windows from cells inside a window.
2. Flashy visual bells may be applied when errors are reported.

https://user-images.githubusercontent.com/2845433/137003476-4fb1fcea-3b76-4edf-8848-69153e99c489.mov

What do you think of

code-cells-header-line '((t :underline t :extend t :inherit font-lock-comment-face)

Looks pretty cool to me and I believe it's a safer default.

[astoff]

Looks better indeed, but what do you think of an overline instead? To me the comment marker is part of the cell, so it makes sense to put the division above it.

[memeplex]

That works for me also.

[memeplex]

To me the comment marker is part of the cell, so it makes sense to put the division above it.

This makes sense. I was thinking of the text as a title to underline, but many times there is no text at all, so your suggestion is better.