While it is not presented in the tutorial, it turns out Shift-X in Helix is the "select current line without surprises" command that normal-X _should be._
I want to learn key sequences for common operations and not worry about whether (in this case) x d will delete the _next_ line because the current line happens to be blank. So, shift-X for me.
(cc
@dpiponiDan Piponi)