And I find that, hilariously, my visual system actively fights the merging of the boxes. When I successfully look at a real stereogram, there is a point at which my eyes snap into alignment and the stereogram pops out. In this case, it's like the opposite: I try to steer my eyes to the point where the blue and yellow boxes overlap, but if I get close they skitter apart. It's like trying to bring two identical magnetic poles together.

I'm sure this stuff is highly idiosyncratic. Everyone's visual system is different. It develops via visual feedback at early ages and so is different for everyone, sometimes to one's detriment:

http://www.lazyeye.org/

It would probably help if the "+" were a more visible color than white, as it gets lost a bit in the yellow when crossing.

My visual system fought back too. I'm normally able to free-fuse, but in this case my right eye just started hurting.

I don't get a single blue-yellow colour that I'm happy with, instead it either swaps from one to the other and back (patchily), or it holds semi steady as if I was looking at yellow through a blue mesh (or blue through a yellow mesh) but doesn't feel like a single colour, more like a checkerboard grid of two colours.

I get the same checker effect. That, or I see a gradient of blue to yellow. Maybe it's just my LCD.

I've seen the blue-yellow color only once before -- during a cluster headache. Seeing it again did not bring back the fondest of memories.