The new Thunderbolt 3 connectivity standard uses USB Type-C plugs and cables. As per Intel's presentation (if I interpret slides 13-15 correctly), all Thunderbolt ports also support the DisplayPort alternate mode, so that at every Thunderbolt 3 USB-C plug, a DisplayPort monitor can be connected via an adapter.
Some USB-C alternate mode adapters use only some of the wire pairs for alternate purposes, e.g. Apple's AV multiport adapter uses two wire pairs for DisplayPort, and the remaining two for USB 3.1 transmissions at 5 Gbit/s (see the fifth slide of this presentation).
Will those "mixed alternate mode" adapters work on a Thunderbolt port as well, or are USB and DisplayPort mutually exclusive on Intel's Thunderbolt implementation (Alpine Ridge)? What about Thunderbolt ports on external docks?
If this is something that is not specified in either the DisplayPort-over-USB or the Thunderbolt 3 specs, I would also be happy about some anecdotal evidence of a working "mixed-alt-mode" adapter on a Thunderbolt-enabled USB port.
An example for an "alt-mode only" adapter is Google's USB-C to DisplayPort cable (it uses all four lanes for the DisplayPort output, and should therefore definitely work with Thunderbolt 3 ports); an example for a "mixed-alt-mode" adapter is Apple's AV Multiport adapter.