"As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years."
Apparently, my memory is faulty.
There is a novel by Kim Stanley Robinson whose plot revolves around a city that moves along tracks that circle mercury. With the heating and cooling of the tracks, due to the sun, being the force the drives the city, rather than electricity.
It also depicts people walking on the dark side staying just shy of the sun side.
That's a very classical approach to scifi, like those Larry Niven stories (Neutron Star etc) where a lone hero man with no friends, but a spaceship, defeats a high school physics class problem. It turns out reading only books about lone men with no friends doesn't prepare you for real life.
My understanding is that scientists believed it was tidally locked for quite some time (until the mid-1960s, and not confirmed until Mariner 10 in 1974-5): the resonance between its rotation and its orbit (together with limitations of where in its orbit Earth had a good view of its surface) meant that observers routinely saw the same surface features in the same places. Some of that history is discussed here: https://en.wikipedia.org/wiki/Mercury_(planet)#Ground-based_...