I experience the same thing, although I cannot even visualize a fleeting triangle unless I am at the edge of falling asleep. I would describe the focus effect a

I experience the same thing, although I cannot even visualize a fleeting triangle unless I am at the edge of falling asleep. I would describe the focus effect as similar to looking through a pair of binoculars, making the target occupy the entire field of view with an exaggerated sense of depth. I associate it with flow and consciously noticing the effect usually interrupts my focus enough to make it dissipate. I've never found a name for this phenomenon.