Plex HTPC - video is full screen, UI is not

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Weird problem. When running the Plex HTPC player on a second monitor (a 4k tv), the UI just fills a 4th the screen on the bottom left, the rest of the screen is black. Video still plays full screen. Anybody have a fix for this?

See Plex HTPC feedback - #1157 by millercentral and subsequent replies. Reply there if you have more information to add.