Requires using SavedVariables. Also look into using LibWindow, so that the window position gets saved in a relative manner (and also solves the issue of going offscreen when resolution changes).

Doesn't actually require saved variables, since frames registered for drag are saved automatically to a file:
https://wow.gamepedia.com/Layout-local.txt

LibWindow makes this resolution-independent though, which prevents the window from being lost offscreen when resolution is changed.