According to the Copenhagen interpretation, in the double slit experiment photons/electrons act as waves until measured, that's when the wave function collapses and they act as particles. But isn't the photo sensitive detector that the interference pattern is project upon in itself considered a measurement? Why does the measurement has to be only on the slit to collapse the wave function?
There is the so-called "decoherence", which offers a description for the loss of quantum correlations of quantum particles. This loss of quantum correlations is what occurs when you either measure the particles or they in some other way leave a trace (interact with their environment). There are experiments that have been made on this that basically say that the higher the amount of information spread by the particles, the less clear the interference pattern becomes (see here for example: N Kerker et al 2020 New J. Phys. 22 063039). Now, why do we see interference on the screen? Because at that point, the interference has already taken place while the quantum correlation hadn't decohered yet. But another thing to consider: If you actually shoot single particles through a double slit, the whole interference pattern wouldn't be visible after one particle. In that case, the particle's wave function would "collapse" and the particle would show in one spot of the interference pattern (according to the probability described by its wave function). If you do this a lot of times, the full interference pattern will gradually become more visible.