Auto-scroll and speed when a drag hits the screen edge

What it is

The drop often sits outside the current viewport. A finger cannot carry an object past the physical bezel, so when the floating object enters an edge band the container must scroll on the finger’s behalf. Auto-scroll speed decides whether a long-distance drop is a controllable cruise or a fling that throws the target past. It extends drag in space. It is not ordinary list inertia, and it is not panning a canvas.

Why it happens

The band turns the bezel into a continuous command: “keep going this way.” Speed usually grows with depth into the band: slow just inside so nearby slots stay aimable; fast at the limit so a long list can be crossed. If speed is not synced to row height, the insertion gap strobes between items and the drop will not sit still. Direction must follow the band, not the contact’s instantaneous velocity, or a lateral micro-adjust at the edge accidentally accelerates the vertical. Leaving the band should brake within a frame or two, or the object keeps flying after the person has turned back to aim.

Studying it

Place the target slot N screens outside the viewport. Vary band width, the speed curve (linear / exponential / stepped), and max speed. Dependent measures include time to arrive, overshoots that had to reverse, and drops into slots that only existed because the band was scrolling. Also log whether the finger parks in the band “waiting for it to roll” or pecks in and out. Keep long lists and sectioned lists separate: fast scrolling through section headers costs people their structural anchors.

Where it stops holding

On a short list that already shows every slot, auto-scroll is noise, and an edge band turns a near-bottom drop into an accidental page flip. Horizontal drawers and pagers already own the edge for system gestures, so auto-scroll may be stolen by back or page-change. A keyboard shrinking the viewport stacks the band on input accessories and invalidates the speed calibration. Pen and mouse can leave the window and use system drag-and-drop, and may not need edge scrolling.

Applying it

• Map speed to distance-from-edge: about one row per second near the inside, then accelerate deeper. Stop immediately on leaving the band; do not keep inertia.
• Do not let the band cover the last few rows that are themselves drop targets: leave a top and bottom “drop here, don’t scroll” belt.
• Have people drag to a target three screens away. Count overshoots and band entries. Pecking means too fast or no brake; a long dead wait means the band is too thin or the curve too lazy.

Related

• Same group: C3.04.1 Drag may start from a long-press or from an immediate down, with different meanings · C3.04.2 Competition between drag-start slop and scroll-direction discrimination · C3.04.3 The drop target must stay visible during a drag
• Adjacent: C3.06 Fling and inertia · C3.13 Edge swipe gestures
• Search: auto-scroll · edge scrolling · drag and drop