I'm trying to wrap my mind around the crop tool and the its ability to slide the crop area to the sub-pixel level. Why does it function like this, and what happens when you crop a picture to the sub-pixel?
e.g. Let's say you crop a 200×200 picture with the crop selection's height set from the bottom to about 180 and 1/4 of a pixel upwards. It of course isn't going to result in a photo that is 180 and 1/4 of a pixel tall, since Photoshop deals with raster, full-pixels. Does it just take off the extra quarter of a pixel? Or does it resample the image in some way? Which takes me back to the question, why would it function this way, when you're always gong to wind up with an image in exact pixels?
Do you zoom heavily on image to see what is described? When cropping Photoshop will snap to full pixel of course, as already stated above. For more details you may want to visit Photoshop feedback site that is monitored by Adobe employees and engineers https://feedback.photoshop.com/
When you commit the crop it rounds up or down, including or excluding the partial pixels.
Of course you can resample with the crop tool if you set it to do so, by defining a pixel size in the options bar. That size can either be set directly as pixels, or indirectly as print size at a certain ppi.
So maybe they just decided it was easier and more consistent to not have the crop tool snap. I don't know.
If you need to snap to pixels, it's probably better to use the rectangular marquee, which does snap if set to do so in preferences. The marquee can also be directly set to either pixel size or ratio in the options bar.