We have a resident "shadow chaser" who arrived here displaying this behaviour aged 7 months. She is a puppy farm cocker now aged 2 years 3 months. We even tried another rescue for several months but the obsession remained. No matter how good you are at finding an alternative, you will never totally remove the OCD, yes you can reduce it by covering all glass in the house, anything which shines or reflects light, mirrors, keeping the blinds lowered when the sun *should be* streaming in the window and certainly not any nice shiny flooring. Distraction has to be with something which is of much higher interest or value, remember this poor dog is like someone with a heroin addiction they must have that "fix" at all costs, that's why we talk about not "feeding" the obsession.
You remove all sources you can, then you look for an alternative. No one can tell you what that will be because each shadow chaser will have a different alternative. Some love squeaky toys, agility, clicker training but I would hazard a guess that most respond to extremely high levels of exercise, a tired dog does not chase shadows!
Some people can tolerate and live with it, others find it too distressing and they end up rehoming the dog or worse.
