Sliding a finger between frets of the same string is
usually represented with a straight line connecting
both the notes in the Staff and connecting the numbers
in the TabStaff. Because the fret numbers are on the
same string, Lilypond adds a flat line right on top of
the staff lines. A sloping line connecting the bottom
of one fret number to the top of the other (or vice
versa) would be ideal.
Similarly, in guitar a slide (glissando) can happen
between half steps. In the following to glissando line
is hidden in the staff lines. As in the tab it would
be better if the line would connect the bottom of one
note to the top of the next: