I've searched around but can't find the answer to this, and for me, it's an annoying one.
If I create an object then proceed to duplicate it pressing alt+drag (or alt+shift+drag) and move it, no snapping will occur, not on any line, ruler, grid, pixel, vector etc. It's an free floating object.
Unless I'm doing something wrong, I feel like this function should be added. I often work with UI and want to be able quickly align new elements w/o needing to zoom in and place them again because there's a pixel gap of 1-2 pixels.