When playing media in full screen in Safari, after around 10 minutes if you try to exit full screen or navigate to a different space in Mission Control, playback will stop and the tab will completely lock up showing the beach ball cursor, requiring the tab to be closed to be usable again. This happens consistently with no extensions enabled.
Server Version#: 1.18.4.2171
Player Version#: 4.20.1
Safari Version#: 13.0.4
macOS Version#: 10.15.2
I tried to reproduce this issue on my Mac and was unable to. I tested using both the bundled web client (4.12.3) and the hosted version (4.20.1). The closest I came to having a problem was a 2-3 pause when switching from full-screen back to normal using 4.20.1. I thought for a moment it had locked, but it recovered on its own. For each test I watched ~11 minutes to be on the safe side.
For reference, when I tested I was using:
macOS 10.15.2 (19C57)
Safari 13.0.4 (15608.4.9.1.3)
PMS 1.18.4.2171
Web Clients 4.12.3 and 4.20.1