If you run the ThinLinc client on a HiDPI screen with Wayland and display scaling turned on (for example 200%), the client does not get scaled up. The remote desktop is shown at native resolution, so everything looks tiny.
This is because the ThinLinc client is an X11 application running under Xwayland. Both GNOME and KDE Plasma can give X11 applications the full native resolution instead of upscaling them, so the client creates a session of that size.
One option is to enable scaling in the desktop environment of the ThinLinc session, just as on a local HiDPI screen. This gives the sharpest result, but relies on the applications handling scaling well.
The other option is to have the client’s desktop upscale the client, so that the session gets a lower resolution and is shown enlarged. This is done by changing how Xwayland applications are scaled on the client.
GNOME
Since GNOME 50, native scaling of Xwayland applications is the default. You can override it by telling GNOME to run Xwayland at scale 1:
gsettings set org.gnome.mutter.wayland xwayland-scaling-factor 1
The change takes effect immediately. With the display at 200%, the client now sees half the resolution, the session is created at that size, and GNOME doubles it on screen. To go back to the default behaviour:
gsettings reset org.gnome.mutter.wayland xwayland-scaling-factor
The xwayland-scaling-factor setting was also added in GNOME 50. In GNOME 47 to 49, native Xwayland scaling was an experimental feature that some distributions enabled by default. On those versions, make sure xwayland-native-scaling is not listed in:
gsettings get org.gnome.mutter experimental-features
KDE Plasma
Plasma has had a setting for this since version 5.26. Open System Settings, go to Display & Monitor, and change “Legacy applications (X11)” from “Apply scaling themselves” (the default) to “Scaled by the system”.
NOTE: In both cases the setting applies to all X11 applications on the client, not only ThinLinc.
Since the desktop upscales a lower resolution session, the result will look somewhat blurry compared to native resolution. Better HiDPI support is tracked in 7885 – Session high DPI support, and we will be considering it when we migrate the client to a more modern GUI toolkit down the line.