This is most likely from the manga Steins;Gate - Arc Light of the Point at Infinity (it's a quick read - 6 chapters).

If you're interested in spoilers, it's very likely this scene where Mayuri is in the time machine (you can also see there is another person sitting opposite her) is where [she decides to time- travel back to the Rooftop with Suzuha in order to tell her younger self that she must push Okabe (or in this case, slap him lol) once Okabe returns from his first attempt otherwise his Hououin Kyoma persona will be lost forever. This scene happens briefly while future Mayuri is in the time machine, and not making physical contact with past Mayuri (with the usual reason of preventing a time paradox) and also while Okabe and a younger Suzuha went back in time in their first attempt to stop Kurisu from being killed. At this point, this future version of Mayuri and Suzuha know the fuel in the time machine in this scene can only last for one trip (and not back), and so, being unable to return to their time in the future they willingly take that chance and choose to be lost in time (and space?) forever] (/sp)