Hide default separator when a border separates input from list

- Applies when adjacent section (header, header lines, or list) draws
  border line facing input section

    # No separator below header border
    fzf --style full --input-border none --header foo

    # Separator shown; nothing separates input from list
    fzf --style full --input-border none --header foo --no-header-border

- Evaluated dynamically so change-header/toggle-header update it
- Inline header is drawn inside list border and not adjacent to input;
  decide from section actually next to it
- Separator is kept when a preview window can be at 'next' position,
  right next to the input section

src/options.go

@@ -3696,6 +3696,9 @@ func (opts *Options) noSeparatorLine() bool {
 	if opts.Inputless {
 		return true
 	}
+	// NOTE: This does not know that the default separator can be suppressed at
+	// runtime (see Terminal.separatorLength), so the minimum heights derived
+	// from it can be one line taller than strictly necessary.
 	sep := opts.Separator == nil && !opts.InputBorderShape.Visible() || opts.Separator != nil && len(*opts.Separator) > 0
 	return noSeparatorLine(opts.InfoStyle, sep)
 }