Yes, that rings a bell. I think it might have been to do with this being the minimum version that works for macOS. @bjeurissen might be able to give the specifics, I must admit I can't remember...

I can only remember #2120 where I argued that Qt >=5.5 should be available in all versions of popular linux distributions that are realistically still being used.

That said, there might be reasons to opt for more recent qt versions, e.g. better hdpi support or better dark mode support, ... But as long as it compiles and works properly, I think we can deem it supported..

So I would just look at the oldest linux distribution versions that are still supported by their maintainers and see what Qt version they package to decide a minimal requirement. At least then we are sure it will work out of the box on popular distributions.

There's a lot of precompiler directives for Qt >= 5.4, and one for 5.5. I reckon given it's hard to find anything less than 5.5, we should just state that we expect that, and then for 3.1.0 we can remove all of those directives.

I'm happy enough to mandate Qt≥5.5, it will simplify the OpenGL handling substantially (that's what a lot of those directives will relate to).

We'd need to update the Qt checks in the configure script, and I'dd probably add a compile time check in there somewhere, probably in src/gui/opengl/gl.h (which is included pretty much everywhere GUI related), of the form:

#if QT_VERSION < 0x050500
# error "MRtrix requires Qt version 5.5 or later"
#endif