Use 'grid' to clip segments in geom_text_s(), geom_label_s(), geom_label_pairwise() and geom_text_pairwise().

[aphalo]

In geom_label_pairwise() the segment behind the label box becomes visible when the box fill is transparent. This is a bug.
In geom_text_pairwise() the default position of the text causes no problem, but if centred on the segment the text is crossed out!
In geom_text_s() and geom_label_s() clipping to the "padded text or box" would greatly simplify the code used for drawing segments as they could be drawn from the center of the text or label grobs.