To add a border to separate preview contents explicitly when in preview mode

[rustyhu]

When use the preview mode I sometimes can not tell whether it is in the mode, as when previewing a directory the items in it (listing at rightside) look quite similar to the main view contents (listing at leftside). I think a border line is helpful.

How about it, like this:

not in preview

in preview

[antonmedv]

I think it is possible but I like minimalistic view of walk.

[rustyhu]

For situation like following, or sometimes with a large number of files to list meanwhile to preview, current layout may be not easy for user to see clearly.