> last release was from June 2015 ... no longer maintained

Dude. I'm using software that hasn't been maintained since 1983. You're worried about 2015?

Software from 1983 that's still around probably has less issues than the average unmaintained project from today, if only because of survivorship bias.

I've been using FileMerge "forever". It comes bundled with XCode and can be used in git as "opendiff".

I've used the same. One thing that is odd is the "merge" window that shows the final result of the merge that sits below the left and right pane is set to 0 height by default. I've surprised several colleagues by moving my mouse to their bottom pane of FileMerge and dragging up showing them that 3rd pane.

Just in case anyone involved reads this: I hate how Black Pixel bought Kaleidoscope just to let it rot.

I've been using Kaleidoscope for years... I don't know what feature you could want, maybe audio support? Handles my dev needs already.

Happy user of SourceGear DiffMerge here. It's probably not the most beautiful piece of software on the planet, but it gets the job done.