Input anticipation

Input anticipation

Most interfaces are reactive: the user clicks, the UI responds. Anticipatory interfaces react to intent — where the pointer is going — a beat before the interaction lands. Done on the right element it reads as "this feels expensive"; done on every element it's a laggy gimmick that hurts. This skill implements the three core patterns and, just as importantly, encodes when not to use them.

The whole family is built on one cheap signal: how far is the pointer from this element, and where is it heading? Everything else is turning that signal into opacity, transform, or a prefetch.

Before you build: the two rules that keep this tasteful

These aren't footnotes — they're why the effect works at all. Read them first.

1. Reserve it for 1–2 high-value targets. Every anticipating element runs geometry on every pointermove (dozens of events per second). One AI chat field or one hero CTA: imperceptible. Two hundred buttons each doing their own distance math: you've built a space heater and the page stutters. The effect also means less when everything does it — anticipation signals "this is the thing you came for." Apply it to the primary input/CTA/nav, not the whole page.

2. It's pointer-only, so it must be pure decoration — never function. Keyboard users, touch users, and screen-reader users have no cursor to anticipate, so they get none of this. That's fine as long as the underlying control works completely without it: the field still focuses, the button still clicks, the link still navigates. The trap is letting anticipation become the only path — e.g. a menu that opens only on predicted approach, or a control reachable only by sweeping a mouse near it. That locks out everyone not on a mouse. Rule: anticipation makes the fast path feel faster; it is never the only path. Also honor prefers-reduced-motion for anything that moves.

The shared signal: distance from a point to a rectangle

You almost never want distance to an element's center — you want distance to its nearest edge, which should read as 0 the instant the pointer is anywhere over the element. The clamp trick gives you exactly that:

function distanceToRect(x, y, rect) {
  const dx = Math.max(rect.left - x, 0, x - rect.right);
  const dy = Math.max(rect.top - y, 0, y - rect.bottom);
  return Math.hypot(dx, dy);
}

Why the Math.max(a, 0, b): if the pointer is left of the box, left - x is positive and wins; if right of it, x - right wins; if horizontally inside the box's span, both are negative and 0 wins — no horizontal gap. Same for dy. The result is the gap to the nearest edge, and 0 anywhere inside.

Turn distance into a 0→1 "intent" with a falloff curve:

function intentFrom(distance, radius = 180, exponent = 2) {
  return Math.max(0, 1 - distance / radius) ** exponent;
}

radius is how far away the element starts noticing the pointer. exponent shapes the response: ^1 is linear and feels mushy (the element reacts to faraway movement); ^2 keeps it quiet until the pointer is committed, then ramps fast. The squaring is doing the design work — it makes the response non-linear so distant motion is ignored and near motion is amplified. Default to ^2.

Performance: never drive the hot path through framework state

A pointermove handler fires constantly. Re-rendering a component on every event will cost frames. Read layout and write style straight to the DOM node; reserve state for things that change rarely (slider values, enabled/disabled). Read getBoundingClientRect() inside the handler (it reflects scroll); if you cache it, invalidate on scroll/resize.

Pattern 1 — Proximity focus ring

A ring/glow whose opacity tracks pointer distance. The canonical use is an AI chat input that "wakes up" as you approach. Vanilla:

const field = document.querySelector(".field");   // the element to sense
const ring  = document.querySelector(".ring");    // decorative overlay, aria-hidden

addEventListener("pointermove", (e) => {
  const d = distanceToRect(e.clientX, e.clientY, field.getBoundingClientRect());
  ring.style.opacity = intentFrom(d, 180, 2);
});

.ring {
  position: absolute; inset: -4px; border-radius: inherit;
  pointer-events: none; opacity: 0;
  transition: opacity 90ms linear;            /* smooths the per-event jumps */
  box-shadow: 0 0 0 1px hsl(217 91% 60% / .6),
              0 0 24px 2px hsl(217 91% 60% / .45);
}

Mark the ring aria-hidden="true" — it's pure decoration. Scope the listener to a container instead of window when you have several on a page, and reset to 0 on pointerleave. You can drive more than opacity off the same intent: warm a Send button's border, lift a placeholder. Keep it subtle.

React note: keep intent out of state. Hold refs to the field and ring, write ringRef.current.style.opacity in the handler. Use state only for tunables like radius/exponent, and mirror them into refs so the handler reads current values.

Pattern 2 — Magnetic target

The element leans toward the cursor as it approaches, so the click region effectively grows in the direction of travel. Same proximity value, applied to a transform:

const btn = document.querySelector(".magnetic");
const RANGE = 140, MAX = 14;                       // px sensing radius, px travel

btn.addEventListener("pointermove", (e) => {       // listen on a zone around it
  const r = btn.getBoundingClientRect();
  const cx = r.left + r.width / 2, cy = r.top + r.height / 2;
  const pull = Math.max(0, 1 - Math.hypot(e.clientX - cx, e.clientY - cy) / RANGE);
  const tx = ((e.clientX - cx) / RANGE) * MAX * pull * 2;
  const ty = ((e.clientY - cy) / RANGE) * MAX * pull * 2;
  btn.style.transform = `translate(${tx}px, ${ty}px) scale(${1 + pull * 0.06})`;
});
btn.addEventListener("pointerleave", () => { btn.style.transform = ""; });

.magnetic { transition: transform 220ms cubic-bezier(0.34, 1.56, 0.64, 1); }
@media (prefers-reduced-motion: reduce) { .magnetic { transition: none; } }

Keep MAX small (10–15px). Past that the button feels like it's dodging the cursor, which is the opposite of helpful. The overshoot easing (cubic-bezier(0.34,1.56,0.64,1)) makes the spring-back feel physical. Guard on prefers-reduced-motion — bail out of the handler entirely for users who asked the OS to reduce motion, since this one literally moves things.

Pattern 3 — Trajectory prediction (the useful one)

The strongest anticipation isn't proximity, it's heading: figure out which target the cursor is aimed at and arm it early — highlight it, or kick off the fetch its destination needs, during the ~200ms between intent and click.

Track a smoothed velocity, then for each candidate take the dot product of the heading with the direction to that candidate. A dot near 1 means "aimed straight at it." Require a forward cone so a glancing pass doesn't count, and add a gentle distance penalty so a nearer aligned target wins ties. When the pointer is nearly still there's no heading to read, so predict nothing — don't silently fall back to nearest-target, or every paused cursor lights something up.

Update velocity in the pointermove handler, but run the selection in a single requestAnimationFrame loop reading the latest velocity. That decouples the cost from event frequency and gives you one place to also draw a heading indicator.

let last = null, vx = 0, vy = 0, pointer = null;

zone.addEventListener("pointermove", (e) => {
  if (last) {
    const dt = Math.max(1, e.timeStamp - last.t);
    vx = vx * 0.7 + ((e.clientX - last.x) / dt) * 0.3;   // EMA, px/ms
    vy = vy * 0.7 + ((e.clientY - last.y) / dt) * 0.3;
  }
  last = { x: e.clientX, y: e.clientY, t: e.timeStamp };
  pointer = { x: e.clientX, y: e.clientY };
});

let predicted = null, armAt = 0;
function frame(t) {
  if (pointer) {
    const speed = Math.hypot(vx, vy);
    let best = null, bestScore = -Infinity;
    if (speed > 0.04) {                                   // moving → has a heading
      const nvx = vx / speed, nvy = vy / speed;
      for (const el of targets) {
        const r = el.getBoundingClientRect();
        const dx = r.left + r.width / 2 - pointer.x;
        const dy = r.top + r.height / 2 - pointer.y;
        const d = Math.hypot(dx, dy) || 1;
        const align = (dx / d) * nvx + (dy / d) * nvy;    // -1..1
        const score = align - d / 1600;                   // aligned + near wins
        if (align > 0.62 && score > bestScore) { bestScore = score; best = el; }
      }
    }
    if (best !== predicted) {                             // prediction moved
      highlight(predicted, false); highlight(best, true);
      predicted = best; armAt = t;                        // restart the hold timer
    } else if (best && t - armAt > 160) {
      prefetch(best);                                     // held → confidence → prefetch
    }
  }
  requestAnimationFrame(frame);
}
requestAnimationFrame(frame);

The lesson the demo on seangeng.com learned the hard way: arm only the single current prediction, and clear it the instant the prediction moves. A "prefetched" flag that accumulates across targets ends with everything lit, which destroys the signal — the whole point is that one thing reacts. Make a real prefetch cache idempotent and invisible; keep the visible armed state tied to the live prediction. Reset everything on pointerleave.

This is the genuinely productive version: submenus that open along the path to them, links that prefetch when the user is clearly headed there, a search index that warms as the pointer drifts toward the box. Prefetch is a hint, not a commitment — debounce it (only fire once a target stays predicted for a breath) so a sweep across the row doesn't fire five fetches, and make sure the real click still works if the prefetch hasn't finished.

Putting it together / checklist

When you implement any of these, verify:

• The control works fully without the effect (keyboard, touch, SR).
• Applied to only 1–2 high-value targets, not sprinkled everywhere.
• Hot path writes to the DOM directly; no per-event framework re-render.
• Decorative overlays are aria-hidden; listeners reset on pointerleave.
• prefers-reduced-motion disables anything that moves (patterns 2 and any animated arming in 3).
• Prefetch (pattern 3) is debounced and never blocks the real action.

Credit: the proximity-ring formulation is from @gabriell_lab.