Decoherence only happens at the detector, if it is present. If the detector is absent, then decoherence can only take place at the wall. But if you were to shoot the electron one at a time, you would only see one spot on the wall. It's only when you start sending multiple electrons that the interference pattern emerges (if there is no initial detector). This is consistent with both MWI and the Copenhagen interpretations. For example, here are the results of a double-slit-experiment performed by Dr. Tonomura showing the build-up of an interference pattern of single electrons. Numbers of electrons are 11 (a), 200 (b), 6000 (c), 40000 (d), 140000 (e):
An understanding of the experiment helps better understand what it does and does not entail: https://en.wikipedia.org/wiki/Double-slit_experiment#Interference_of_individual_particles
For further details about the specific experiment by Dr. Tonomura, check out this link: https://web.archive.org/web/20110114170600/http://www.hitachi.com/rd/research/em/doubleslit.html