Comment 16 for bug 1615734

I have the same setup as @ritterkeks in comment #10, and the same issues.

The P50 dock and it's DisplayPort hub is apparently the problem. The mini-DP connector on the laptop itself is working much better (I'm also seeing some OpenGL bugs there on my Lenovo P27 4K monitor (had to enable triple buffering), but that's apparently something else.

When I connect the monitor to either dock's DP ports the monitor's power led goes green like it's receiveng some signal, bit then goes amber into a power safe more. In the meantime the builtin monitor does pretty much the same, except it also shows some picture for a fraction of a second. This repeats sometimes forever (till I time out and pull the monitor's DP cable from the dock), or sometimes the xserver gives up and falls back to the builtin screen itself.
It happenede that it was sometimes successful and I saw a proper picture on both screens, but that was far from stable/usable. After a bit of desperate searching I've fould that the dock itself has an upgradable firmware. The current latest version is 2.33. After jumping through hoops of installing windows (thanks a ton, Lenovo) I've managed to flash the dock. To my surprise, the monitor adoption got worse and since then, I can't ever see any picture on the 4K monitor via the dock at all (but compared to the previous unusable state it's really no big deal). With Windows it works fine, though, so I'm sure the firmware upgrade didn't really brick it.

For those who want to try flashing the dock too 3:), you can find the binary here: https://support.lenovo.com/us/en/accessories/B697B003-94C2-418E-914F-23C201052DB1
Note the flashing utility is itself a "brilliant masterpiece" and most of the time it does nothing after you run it. At least apparently, but it does produce an "update.txt" file in the CWD, which you can examine for a status message. Also be sure to connect something to the dock when flashing it, as otherwise the utility shows it can't connect and bails out. So much facepalm.