MKVToolNix GUI v15.0.0
Ubuntu 18.04.1 LTS
3840x2160 (16:9) resolution Dell XPS 15 Scale 200%
GUI is too small to be legible in 4k+ DPI monitors. Does not scale like other Ubuntu apps with global scale setting.
Note that titlebar is about 12 px font. Anything smaller is too small to read.