I liked Sublime Merge when I used it a few years ago, mainly because it's available on Linux. There were some things about the UI that were bugging me however. Nowadays I'm pretty happy with Fork (available for Windows & macOS).
I dislike that too, but one reason is that they're actually using the platform native UI toolkits (making it wonderfully well integrated and snappy). Adding another platform would be a lot of work and the authors don't believe that Linux even has a clear platform native UI toolkit (though imo GTK would be a safe choice).