External display adapters support
Background: all USBC MacBooks support external screens, however it is not that easy to get a picture on Filevault login / Recovery / Boot menu. The problem is that USBC Display negotiation starts only after GPU driver is loaded, which will never happen if you are not able to click on your password field and type password. The common technique is to physically disconnect internal screen and to fiddle with connectors till something works. However on some T2 models it looks like even then, there might be no external picture. This issue looks pretty obscure and funny but i see no better way but to test every device i have my hands on and to build up a table with known-good cable combinations. To do the trick with internal screen connected, connect external keyboard/mouse, hard shut down and then turn on the device and immidiately close the lid(it should chime with lid closed). Boot logo should appear on external screen even before the Filevault login thus indicating you are able to backup data before you fix the screen.
MacBook 13" (iGPU)
USBC-DP (Cable Matters USB C Mini DisplayPort) as DP1 in table(add your cables)
USBC-HDMI (Uni USB C to HDMI) as HDMI1
USBC HUB (Anker 8-in-1 USB-C Hub) + normal HDMI-HDMI Cable as HUB1
| Device | USBC > DisplayPort | USBC > HDMI | USBC > USBC | USBC HUB with HDMI/DP/Etc |
| MacBook 2016-2017A1708/A1706 | DP1 | X | X | HUB1 |
| MacBook 2018-2019 A1989
(Same results with internal display disconnected) |
DP1 | X | X | HUB1 |