Apparently macOS has a flag that seems to magically make most of ThinLinc be high DPI without any noticeable side effects. This flag was recently merged into TigerVNC: https://github.com/TigerVNC/tigervnc/pull/371 I tried adding it to tlclient as well and I could not see any issues. It doesn't fix everything, but close. And given the low effort I'd say this is worthwhile to fix.
The flag has been added to both tlclient and vncviewer now. Unfortunately, I cannot find a OS X system with a high DPI screen to test with, so I'm leaving that to the tester. Time report on bug 6153.
(In reply to comment #5) > The flag has been added to both tlclient and vncviewer now. Unfortunately, I > cannot find a OS X system with a high DPI screen to test with, so I'm leaving > that to the tester. Actually, I was able to test using the method described here: https://www.tekrevue.com/tip/hidpi-mode-os-x/
The client interface looks great! Tested with build 5369 on Mac OS 10.12.