Even if the right section of the door is latched and used rarely, as long as it is opened in case of necessity, it would still be considered a door, and the mezuzah should not be put on its edge.
However, the permanently fixed glass section of a sliding door is just a glass extension of the wall, and a mezuzah may be put on its end (when it is deemed to be the right post).[A]
 Agur B’ohalecha 22:9; Cf. Chovas HaDar 3:3:8-9, who rules that one should put the mezuzah on the fixed door in a case that the door is only opened to bring in packages.