"whose one and only character trait is her despondency"
… she is, and is meant to be portrayed as, a very broken and hopeless woman who has, in a short period of time:
Been suffering from (presumably) depression which she has ceased to take her medication to help deal with.
Had her daughter abandon her rather than…

A movie that actually touches on, if not entirely gracefully handles, the issues of depression, abandonment, and self-care? This is "unacceptable" and "horrible"? If anything, the movie takes a stance -against- Rebecca for regarding her mother as a burden and bailing on her! Sophie is a person suffering from a lot of…

In short: Diana is capable to -some- extent of screwing with lights, though it's never really clear or even fully consistent (there's a scene where the lights flicker and you -think- Diana is preparing to go after Rebecca, but then the light… stabilizes? Maybe it was just a warning, but kinda jarring to see that and…