We're going to introduce a short tutorial that makes users aware of the interaction when they first open the camera so that should make it more discoverable. Another thing that isn't obvious yet is you can deselect all the images by shaking the device! We'll make that obvious in the tutorial too.
Vertical scrolling isn't affected. A horizontal slide of your finger will activate it and then you can slide over any photo you want to select (horizontal, diagonal and vertical). Once you lift your finger you can scroll down as you normally would. We take into account the angle at which you're scrolling so it is possible to select say 3 photos in the same column by sliding your finger over the top one at an angle and then sliding down. Try it out if you have an iOS device.
Maybe the title is a little confusing. I didn't mean for it to be giving instructions, just a comparison of the interactions on desktop (click and drag) and mobile (tap and slide).
OP here. It's actually a single mode. Since the view doesn't have a horizontal scroll, the selection is activated as soon as you slide your finger horizontally and will continue (horizontal or vertical) until you lift your finger. Of course this means that only the visible cells on screen can be selected in one go before you'd have to release (it will keep the selection) and then scroll down before selecting another group. Hope that clears it up!