I clicked on https://vector.im/develop/#/room/#purple:matrix.org/$1451504230525382RHTLB:matrix.org in Safari, which was logged into a non-guest account not joined to #purple:matrix.org already. It showed me a 'join this room' prompt; i clicked join; it got stuck on an infinite spinner in the RoomPreviewBar. Switching away from the room and back again fixed it, but then I had to re-load the URL to actually get the context link up.