Snapping

Nobody can drop a box on an exact coordinate with a mouse, and no design would be better for it if they could. Snapping takes the intention and discards the tremor: while the object is inside a tolerance of some reference, its position is quietly replaced by the reference’s own. What it snaps to is the interesting part, because the references form a hierarchy. A fixed grid is the coarsest. Guides a person placed themselves are next. Then the edges, centres, and baselines of the neighbouring objects, which is what makes a diagram tool feel like it understands the drawing rather than just the canvas.

The tolerance is measured in screen pixels, not in document units, which is the detail people get wrong first. Snap at a fixed model distance and the behaviour changes character as the reader zooms: irresistible when zoomed out, unreachable when zoomed in. Keep the catch radius constant on screen, somewhere around 5 to 10 pixels, and it feels the same at every scale. The other classic mistake is snapping the pointer instead of the object. The reader is dragging a shape, so what has to land on the line is the shape’s edge or centre, and every candidate the shape offers should be tested against every nearby reference so that a corner, a midpoint, or a baseline can each be what catches.

Show your work while the drag is live. Snapping that only happens on release is a lottery, so tools draw the alignment as it is found: the guide the object is about to lock onto lights up, an outline shows where the drop will land, and something (a distance badge, a matching gap marker) says why that position was chosen. A snap that happens silently and invisibly reads as the object refusing to go where it was put.

Which brings up the escape hatch. Snapping is a guess about intent, and it is sometimes wrong, so every serious tool lets a reader overrule it by holding a modifier during the drag or turning the grid off outright. Without that, the last few pixels of a layout become impossible, and the feature that was supposed to make alignment effortless is the thing standing in the way.