Spring loading

The Finder has done this since the early nineties and still does: pick a file up, hold it over a folder without letting go, and after a moment the folder opens under the pointer. Hold it over a folder in there and that one opens too, so a drag can descend three levels and land where it was actually going. When you release, the windows the drag sprang open close themselves again, because they were opened by the gesture rather than by you. macOS even lets a person skip the wait by tapping the trackpad while holding, and exposes the delay as a slider in system settings, which tells you how contested the right number is.

Everything about the pattern is really a fix for one problem: a drag is a modal gesture with no room in it for navigation. Once the mouse button is down you cannot click a disclosure triangle, scroll with a second hand, or open a tab, so any destination that is not already on screen is unreachable without dropping the item somewhere temporary and picking it up again. Spring loading turns dwell time, the one thing a drag can still express, into a navigation command. That is why the same behaviour appears far beyond folders: dragging over a collapsed node in a tree expands it, over a browser tab switches to it, over a collapsed accordion section opens it, and over the edge of a scrolling list scrolls it.

The dwell has to be paid for while it runs, exactly as a long press does, or the delay is indistinguishable from nothing happening. A filling ring, a growing highlight, or the folder icon flicking open all work; what does not work is silence followed by a sudden change of context under a pointer that is holding something. The delay itself is a compromise between two failures. Too short and every folder the drag crosses on its way somewhere else flies open, which is disorienting and slow. Too long and nobody discovers the behaviour at all. Whatever the number, leaving the target before it elapses has to cancel cleanly and reset the countdown, since crossing a folder is far more common than aiming at one.

Long press is the neighbouring gesture and the contrast worth holding onto: a long press is a press held in place with nothing in hand, and it usually reveals actions for the thing underneath. Spring loading is a hold that happens inside an existing drag, and it changes where the drag can go rather than what it can do. Both are invisible, both are timing based, and both therefore need an alternative for people who cannot hold a pointer steady for half a second. For a drag that has to traverse, the honest alternative is not a better dwell: it is cut and paste, a move-to command, or a destination picker, which is also the only route a keyboard has.