Modern monitors often have very high DPI which requires special handling in applications as they need to render things at a higher resolution. These applications therefore need to be able to tell what DPI to target.
We would like to full support these monitors in ThinLinc as well, allowing applications to fully use the extra detail these monitors provide. Currently we assume a "standard" DPI in ThinLinc and have no support for these high DPI modes.
Supporting this in ThinLinc adds some extra problems since a session can be resumed from different devices, with different DPIs. The session would need to dynamically reconfigure itself.
The major blocker for this is poor support for DPI unaware applications in Linux. On Windows and macOS applications are scaled by the OS if they don't signal that they can handle these high DPI monitors. On Linux there is no such handling so these applications will look tiny instead. Not even Wayland has had the foresight to handle this case.
There was some basic work done here on improving support in X11:
Can't see much happening since then though.
There is also a discussion here about handling older applications in Wayland:
The work done in bug 7926 included adding partial HiDPI support to the profile chooser. Both profile screenshots and icons now look sharp with scaling turned up.
What remains to be fixed now in terms of profile chooser HiDPI support is the bottom ThinLinc logo which still looks blurry with higher scaling factors.