Pick any application on a PC, Mac, Android (except Plex web app). Open it and look in the upper right hand corner of that window. Maybe even open a few... Check that upper right corner.
Now open the Plex Web App. And recall what used to be in that upper right corner vs. what's in the right hand corner now.
I'm still in the habit of going to the upper right to close a currently playing show or movie and going to the lower right to go or leave full screen.
Please find whoever thought this was a good idea and dump a bunch of glitter into his office chair for me.