Long lists must auto-scroll while dragging
Aliases: edge scrolling · drag auto scroll
What it is
The slot you want is often off-screen: moving item 2 after item 80 cannot require letting go to scroll, because release ends the drag. Auto-scroll means that holding an object near the viewport edge moves the list in that direction so distant slots enter drop range. It does not explain whether pickup looks like a grab, and it does not explain how people without a pointer reorder.
Why it happens
Drag occupies the pointer or finger that would have scrolled. You cannot hold an item and drag a scrollbar at once. Without edge scrolling, reachable slots lock to one screen; long-list reorder is structurally unfinished. The workaround—drop, scroll, grab again—leaves the item in the wrong place after the first drop. The edge band is a spatial threshold: too thin, and it is hard to stay in the trigger; too wide, and fine placement inside the screen starts the list moving, so the target slides out from under the pointer. Speed must scale: a crawl makes eighty items take forever; a rush overshoots. Nested scroll (page and list both move) has to pick the right container, or a reorder drag pans the whole page and slot relations collapse.
Studying it
Use a list longer than two screens. Move an item from the first screen next to a neighbor on the last. Compare no auto-scroll, constant edge speed, and speed that grows as the pointer nears the edge.
Independent variables: trigger-band width, speed curve, whether the dragged object stays under the pointer, which nested scroller moves. Dependent variables: whether the task completes, forced mid-task drops, overshoot-and-return counts, false auto-scrolls during in-screen nudges.
If the lab allows keyboard scrolling as a helper, auto-scroll looks optional. On touch, record whether the system steals the finger at the physical edge. Mean completion time alone misleads: some people drop-and-scroll, so the task “finishes” after the mechanism already failed.
Where it stops holding
When every item fits one screen, auto-scroll is noise and should be off. Horizontal boards scroll on the x-edge; vertical-list rules do not copy over. Infinite lists that have not fetched the region yet must mark “loading” rather than pretend slots exist. Enable auto-scroll only in reorder mode; a browse-time drag toward the edge should not pan the page, or it collides with back gestures and system drawers.
Applying it
- Start scrolling when the drag enters a stated edge band; faster nearer the edge; stop on leaving the band.
- Keep the dragged item under the pointer; do not let the list move while the item sits in the middle of the screen.
- Stop at the end of the list with an “end of list” edge cue; do not spin in empty space.
- Verify: drag a first-screen item toward the bottom edge; later items should appear while the item stays under the pointer. Nudging in the middle of the screen should not move the list. Drag to the true end and confirm it stops.