Your screen resolution determines how your web browser scales the webpage, which then determines what resolution Scratch will render the project at, and a bug means that small enough sprites will not register collisions at low resolutions. So far I could only get the effect to distinguish between HD and 4K resolutions - any less and browsers stop trying to scale the website down. I thought 1440p might have been possible, but the browser doesn't bother scaling it, so this seems to be a 4K specific thing.
