We're apparently the only people around who have random videos of our kids doing various things shot with random phones or cameras that don't always get the rotation right.
Because I just went through several different video players until I found one that allowed me to rotate the video to be right side up.
smplayer for the win.
But that's after trying too many alternatives.
Even VLC, that does pretty much anything and has an overly fancy video geometry thing that allows you to rotate the video by an arbitrary amount gets it completely wrong. Not only does it make it unnecessarily hard to actually do a simple 90° rotation (because obviously you want an analog input thing that makes it possible to rotate by something like 82°, but no obvious way to just fix a vertical video?), but then it keeps the end result within the original aspect ratio box so that the skewed end result is wrongly cropped. I'm sure you can use another setting to edit that aspect ratio too, but after that amount of crazy who cares any more?
And the fact that we must be the only ones to ever encounter this is odd, since google actually finds a lot of people asking for this (usually with the answer being "use ffmpeg to re-encode the whole video correctly", which is one of those answers that are "technically correct, but incredibly stupid").
I'm sure there are plugins for other players, but I would like to congratulate the smplayer developers for just making it easy for people. And it even seems to remember the setting (per video!) so that once you've corrected one video you don't have to correct it over and over again.