Server Version#: QNAP App Version 1.19.1.2701
Player Version#: MacOS App Version 1.8.2.1192-89432618
The slider for changing the size of posters / thumbnails is actually not doin’ a thing. The sizes don’t change.
There’s an early release version of 1.9.0 in this thread which seems to scale fine when the slider is moved.
