Dock magnification

Every item in the row is given a size that is a function of one number: how far its resting centre is from the pointer. Close means large, far means unchanged, and in between there is a smooth falloff, usually a bell curve a hundred or so pixels wide. Nothing is hovered in the binary sense. The effect is continuous, which is why the row bulges rather than blinking one item bigger, and it is what separates this from an ordinary hover state. Mac OS X shipped it on the dock in 2001 and it has been copied into web docks, toolbars and emoji pickers ever since.

The argument for it is density. A row of thirty small icons is unreadable at rest and unclickable at rest, and magnification lets it be both compact and inspectable without a menu opening or a tooltip firing. The argument against it is that the thing you were aiming at moves. Fitts’s law says a bigger target is faster to acquire, and the item under the pointer is certainly bigger; but the same scaling that grew it also pushed its neighbours outward, so the icon you started moving toward is no longer where it was when you started. That is the whole complaint, and it is why people who love the effect and people who turn it off are usually describing different tasks: one is browsing a row, the other is hitting a known target.

That makes the details unusually load-bearing. The row has to grow from a fixed edge, so a dock at the bottom of the screen grows upward and never pushes the content above it, and the item under the pointer should stay under the pointer rather than sliding out from beneath it, which means laying the row out from the pointer rather than simply widening each item where it sits. The falloff should reach two or three neighbours, since a curve too narrow reads as a single item popping and one too wide magnifies everything and therefore nothing. And magnification is not identification: the icon is still an icon, so a dock that relies on the swell to tell you what a thing is has skipped the label.

The nearest neighbour on this site is the magnetic button, which is the same input, pointer distance, spent on a single control that leans toward the cursor instead of a row that reshapes around it. The general information visualisation technique of showing the region under the pointer at full detail while compressing the rest is a fisheye view, and the surface this particular one lives on is a dock. Neither exists here yet. Worth saying plainly: there is no pointer on a touch screen, so this whole family of effects is desktop-only ornament, and a design that needs it to be usable is not usable on a phone.