racket/gui: double-check size on attmpt to set editor scroll

A scroll request could fail if a resize notification is still
in flight (i.e., queued but not yet executed), so double-check
the size on a scroll request.

Closes PR 13582
file/untar: handle unknown extensions by skipping

Also, add/improve logging.
file/untgz: allow extra bytes after tar content

Closes PR 13641
Closes PR 13644