Think of measurement as the application of constraints. The slits are a form of weak measurement. This produces a loose constraint on the state of the electro

Think of measurement as the application of constraints. The slits are a form of weak measurement. This produces a loose constraint on the state of the electron - within the bounds of that constraint there are still many possible states in which it can exist. Depending on the type of measuring device, the constraint applied can be very tight, up to the limits of the uncertainty principle.