I am international STEM PhD student in the UK.
My advisor asked me to try and attempt a question that he has been thinking about for more than five years. He was always very encouraging, but wasn't confident that I would be able to solve it. I wasn't very confident either.
It turned out to be much simpler than expected (and the simplicity of the solution was perhaps his blind spot). I managed to solve it. Since I told him about my solution, he has turned markedly cold towards me, hardly ever replying to emails and appearing a little aloof/defensive during meetings.
I am not a mind-reader, and hence wouldn't like to speculate on why he is acting this way. What are some things that I could do to make him like me again/not screw up my job application? He has collaborators at the very top places, and I'd love to do a postdoc with them.
I have thought about offering him co-authorship, playing up his contribution to the project, etc. But all of these attempts could backfire, as he may feel insulted, etc.
Like I said, I think he is in general a very decent person, and has always been encouraging towards me.