value: Ultra
"use client";
import { useState } from "react";Installation
$ pnpm dlx shadcn@latest add https://motif-ui.vercel.app/r/strength-select.json
The item ships one keyframe, strength-mote, in its css — shadcn add writes it into
your stylesheet along with the component. There is no animation library in the chain: a
translate across a box is a CSS animation, and the component has no reason to carry a
runtime for it.
Usage
import { StrengthSelect } from "@/components/ui/strength-select";
export function Defaults() {
const [strength, setStrength] = useState("Medium");
return (
<div className="space-y-2">
<div className="font-medium text-muted-foreground text-xs">Strength</div>
<StrengthSelect
aria-label="Strength"
onChange={setStrength}
value={strength}
/>
</div>
);
}The heading is yours
The control draws no label. A field's label belongs to the form around it — above, beside,
in a <legend> — and that is layout the caller owns, not something a select should decide
for the page it lands in. So the component renders the track and its own level names, and
no heading above them.
That leaves one line of wiring: a heading you write is plain text, so pass aria-label
with the same words and the name reaches a screen reader too.
A slider that snaps to names
The duration slider is dragged because a duration is a length: every point on its track is
a number of seconds, and the number is the value. Strength is not that. Its levels have
names, and there is nothing between High and Extra high — no value sits at 70% of the
way there.
So the gesture is a slider's and the value is a list's. The pointer runs freely along the track, and the seat it is standing in is the level it lands on. That is where the snap comes from: not from a continuous number being rounded to a step, but from the track being divided into seats that each already have a name. The two controls are the same shape because they sit in the same column of the same form; what differs is that the duration slider's number is the answer and this one's index is only the spelling of it.
The seats are what make the drag honest, and they are the same partition the names below
the track use: a fifth of the track is Low because the word Low sits under that fifth.
Snapping to the nearest edge instead would put Low a fifth of the way across the track,
which is where the word Medium is.
The fill is the amount
Each level owns a seat of the track, so Low fills one fifth of it and Ultra fills all
five. The amount is drawn as an amount, which is what makes the levels comparable at a
glance: two levels apart is twice the fill, and the empty remainder of the track is what a
higher level would buy.
The fill's leading edge is the boundary of the chosen seat, and the playhead stands on it — the same bar the duration slider carries, in the same place. So the level is told twice over and neither half has to be decoded: the length says how much, and the bold name under the track says which.
The edge is the seat's far edge, which means the playhead ends up as much as one seat ahead of the pointer that put it there: press the first fifth and the fill runs to the end of the first fifth. That is the price of "the fill is the amount" — the seat is what you press, and the length is what you bought — and the seat tint following the pointer is what keeps the two readable while you drag.
motion-reduce:transition-none parks the fill for anyone who has asked for less movement.
Only the ceiling moves
The top level — wherever the list ends, three levels or nine — carries a slow drift of motes across its fill. Every level below it is still. Five levels do not need five animations: motion in a form is noise, and a control that is always moving stops being read as a control. Giving it to the one level that costs the most buys the ceiling a character the other four do not have, and it costs the levels below nothing.
It is slow on purpose. A mote crosses the fill in 3.4–5 seconds, which reads as drift; the
first version crossed in under a second and read as a flicker you had to look away from.
Each mote starts partway through its own crossing — a negative animation-delay, which is
how CSS says an animation has already been running — so the row is spread across the fill
at every instant. Their crossing times differ as well, so the spread keeps shifting instead
of settling into a pattern.
The first version got this wrong, and the bug is worth keeping written down: it staggered the motes by a positive delay from a shared start, so all twelve crossed as one clump and the bar sat empty between passes — half a minute of watching could show no mote at all. Phasing is the whole point of a drift; a stagger is not a phase.
The placement is worked out from each mote's index by a hash rather than from
Math.random, so the server and the client draw the same row and none of it has to be
generated in an effect. The hash is integer arithmetic on purpose. Math.sin looks like a
harmless way to spread points — it is what the first version used — but it is not specified
to the last bit, and the browser's engine disagrees with the server's a few ulps out. Those
digits went straight into the style attribute: the server wrote
animation:strength-mote 3.491549070959445s where the browser computed
3.4915490709710864s, and React compares that attribute as text, so it called the two
different rows a hydration mismatch. Math.imul is exact integer multiplication, so an
index gives the same number in every engine.
Each crossing and each height is then rounded — a hundredth of a second, a tenth of a percent — which is finer than the drift can show and keeps a float's full expansion out of the markup.
Each mote's carrier spans the fill, so 100% of its travel is the fill's own width:
nothing is measured in pixels, and the drift works the same whatever the level's fill is
worth.
Under motion-reduce the drift is not drawn at all — not paused, not stilled: Low
through the top level are the one control they always were.
Every name is a target
The names sit under the track, one per seat, and pressing one chooses its level. The heading above the control is a label, so it does nothing when pressed; these are not labels, so they do — the level you read is the level you press.
The whole control is the pointer's surface, so a press on a name is resolved the same way a drag is: the x lands in a seat, and that seat's level is the one you get. The word and the fifth of the track above it cannot disagree, because they are the same partition being asked.
What they are not is a second set of tab stops. A drag, a press on a name, a press on the track and an arrow key all resolve to a seat, and five separately focusable names would ask a keyboard user to walk six stops to describe one value. The focus belongs to the track and the arrows move the selection — so a level is never focused without being chosen.
The tint is what keeps the pointer and the names in step while you drag: the seat under the
pointer lights and its name comes forward, before anything is chosen. The light is the
foreground token at 5% rather than muted or accent — in light mode those two are the
same lightness as the fill, so a muted hover would disappear on the part of the track the
selection already covers. An
outline was the other candidate and lost: a ring the height of the track reads as a box
floating in it rather than as a seat being chosen. The highlight's own corners follow the
same reasoning — it carries the track's radius on the outside of the row and is square on
every edge that meets another seat, because a rounded corner there would poke past the
straight boundary the fill is drawn with.
Any ordered list
options is any list of level names, weakest first; the count sets the segments, and the
component never parses what the names mean. Keep them ascending — the first entry is the
one the track treats as the smallest, and the last is the one that drifts.
value: Deep
Accessibility
The track is a role="slider" named by aria-label, with aria-valuemin, aria-valuemax
and aria-valuenow taken over the index of the level, and aria-valuetext carrying the
name. The index is what a slider is required to expose; the name is what the level actually
is, and it is what gets announced — this is the case aria-valuetext exists for, a value
that is chosen from a range and read as a word.
It is one tab stop. ↑/→ move one level stronger and ↓/← one weaker, Home and
End go to the ends, and the focus stays on the track while the selection moves. The seat
names are aria-hidden — the level they spell is already in aria-valuetext, and reading
five names out around the slider would say the same thing twice. So are the fill and the
motes, which carry nothing the value does not.
touch-none stops the browser from claiming the horizontal drag as a scroll. Unlike the
duration slider, nothing is held back for a release: a drag commits each level as it
crosses it, because each crossing is already a whole named level rather than an
intermediate number.
Component source
"use client";
import { useState } from "react";
import type { KeyboardEvent, PointerEvent } from "react";
import { cn } from "@/lib/utils";
/* -- The seats ----------------------------------------------------------------
* Strength is a level, not a length: the stops have names, and nothing exists between
* them. So the track takes a slider's gesture without a slider's value — the pointer runs
* freely along it and the seat it is standing in is the level it lands on. The snap comes
* from the seats rather than from a length rounded to a step, which is why a press on a
* name under the track and a drag across it agree: both are asking the same partition.
*
* Each level owns a seat, and the fill — the seats up to and including the chosen one — is
* the amount, drawn as an amount, with the playhead standing on its leading edge. That
* edge is the chosen seat's boundary, so the playhead sits up to one seat ahead of the
* pointer that chose it: the seat is what you press, and the level is told once, as a
* length.
*
* Only the top level moves. A level is a name, and five of them do not need five
* animations: the ceiling is the one level worth saying something extra about, and leaving
* the levels below it still keeps the control usable in a form rather than decorative.
* --------------------------------------------------------------------------- */
/** Levels offered when the caller does not pass their own, weakest first. */
const DEFAULT_OPTIONS = [
"Low",
"Medium",
"High",
"Extra high",
"Ultra",
] as const;
/** How many motes cross the fill of the top level. Enough to read as drift, few enough
* that the eye is not counting them. */
const MOTE_COUNT = 12;
/** How far the playhead's bar is held inside the track, so a bar at either end is a whole
* bar rather than half of one. The duration slider holds its own back by the same amount:
* the two controls stand in the same column of the same form. */
const PLAYHEAD_INSET = 2;
/**
* The corners a seat's highlight carries. It is a slice of the track, so it is square on
* every edge that meets another seat and wears the track's own radius only on the outside of
* the row: the first seat's left corners, the last seat's right ones. Rounded everywhere it
* reads as a pill floating in the track, and where a seat ends at the fill's leading edge
* that corner pokes past the straight boundary the amount is drawn with.
*/
const seatRadius = (index: number, count: number) => {
const isFirst = index === 0;
const isLast = index === count - 1;
if (isFirst && isLast) {
return "rounded-lg";
}
if (isFirst) {
return "rounded-l-lg";
}
if (isLast) {
return "rounded-r-lg";
}
return "";
};
/** A signed 32-bit product as its unsigned value, so the halves below can be taken by
* dividing rather than by a bitwise shift — this project's lint forbids those. */
const unsigned = (value: number) => (value < 0 ? value + 4_294_967_296 : value);
/** The top half of a 32-bit value, as a whole number in `[0, 65_536)`. */
const upperHalf = (value: number) => Math.floor(unsigned(value) / 65_536);
/**
* A stable pseudo-random in `[0, 1)`, from a mote's index rather than from `Math.random`,
* so the server and the client draw the same row and nothing has to be generated in an
* effect.
*
* Integers only, and that is the whole point. `Math.sin` is not specified to the last bit:
* the browser's engine and the one rendering on the server disagree a few ulps out, and a
* hash built on it wrote that disagreement straight into the `style` attribute —
* `animation:strength-mote 3.491549070959445s` on the server against
* `3.4915490709710864s` in the browser — which React reads as a hydration mismatch, since
* it compares that attribute as text. `Math.imul` is exact, so this is the same number in
* every engine.
*
* Multiply, keep the top half, multiply again: discarding the low half between rounds is
* what breaks the constant step two neighbouring indices would otherwise share, so the
* twelve motes read as scattered rather than as a run of near-identical values.
*/
const noise = (seed: number) => {
const first = upperHalf(Math.imul(seed + 1, 2_654_435_761));
return upperHalf(Math.imul(first + 1, 2_246_822_519)) / 65_536;
};
/** A number at a fixed precision. Two places is a hundredth of a second on a crossing and
* a tenth of a percent on a height — finer than the drift can show, and short enough that
* the `style` attribute carries a string rather than a float's full expansion. */
const rounded = (value: number, places: number) =>
Number(value.toFixed(places));
/**
* The motes' places, worked out once. Each starts partway through its own crossing — a
* negative `animation-delay` is how CSS says "this animation has already been running for
* a while" — so the row is spread across the fill at every instant rather than crossing as
* one clump. Their crossing times differ as well, so the spread keeps shifting instead of
* settling into a pattern.
*/
const MOTES = Array.from({ length: MOTE_COUNT }, (_, index) => {
const duration = rounded(3.4 + noise(index + 1) * 1.6, 2);
return {
delay: -rounded(duration * ((index + 0.5) / MOTE_COUNT), 2),
duration,
id: `mote-${index}`,
top: rounded(12 + noise(index + 21) * 76, 1),
};
});
/**
* Where the playhead stands for a percentage of the track, clamped a whole bar inside it.
* The fill and the seats use the percentage itself: they are the measure, and a measure
* that stopped short of its own ends would put the playhead a few pixels off the boundary
* it is supposed to be standing on.
*/
const playheadLeft = (pct: number) =>
`clamp(${PLAYHEAD_INSET}px, ${pct}%, calc(100% - ${PLAYHEAD_INSET}px))`;
export interface StrengthSelectProps {
/**
* Names the track for assistive tech, e.g. `"Strength"` or `"Reasoning effort"`. The
* heading above the control is the caller's to draw — the component ships no label,
* because a field's label belongs to the form around it.
*/
"aria-label"?: string;
className?: string;
/**
* Called with the chosen level as soon as the pointer or a key lands on one. A drag
* calls it once per seat it crosses and never for a point between two of them — every
* crossing is a whole named level rather than an intermediate number — so an owning form
* sees a handful of settled values and no stream of in-flight ones.
*/
onChange: (value: string) => void;
/** Levels to offer, weakest first. Five is the designed-for count. */
options?: readonly string[];
/** The level currently in effect. */
value: string;
}
/**
* Strength as a track you drag: the playhead rides the fill's leading edge, and the seat
* the pointer is in is the level it lands on. The level names sit under the track, one per
* seat, where they can be read and pressed. The three ways of choosing are one control —
* a drag, a press and an arrow key all resolve to a seat.
*/
export const StrengthSelect = ({
"aria-label": ariaLabel,
className,
onChange,
options = DEFAULT_OPTIONS,
value,
}: StrengthSelectProps) => {
const [hoveredSeat, setHoveredSeat] = useState<number | null>(null);
const [isDragging, setIsDragging] = useState(false);
const selectedIndex = options.indexOf(value);
const colCount = Math.max(1, options.length);
// The fill is the amount chosen, so the first seat fills a fifth of the track and the
// last fills all of it. A value outside the list draws no fill at all.
const filledPct =
selectedIndex === -1 ? 0 : ((selectedIndex + 1) / colCount) * 100;
// The top of the list, wherever the list ends — three levels or nine.
const isTop = selectedIndex !== -1 && selectedIndex === colCount - 1;
/**
* The seat a pointer is standing in. The seats are equal columns of the track and the
* names under them are the same columns, so a press on a name picks that name's level.
*
* It is the inverse of the fill's mapping and deliberately not its mirror: the fill is a
* length that ends on the chosen seat's far edge, while a point is a seat. Snapping to
* the nearest edge instead would put the pointer on `Low` at a fifth of the way across
* the track, which is where the name `Medium` sits.
*/
const seatAt = (event: PointerEvent<HTMLDivElement>) => {
const rect = event.currentTarget.getBoundingClientRect();
if (rect.width <= 0) {
return null;
}
// The track's right edge is the far edge of the last seat, so a pointer held against
// it lands on the last level rather than one past it.
const pct = Math.min(
1,
Math.max(0, (event.clientX - rect.left) / rect.width)
);
return Math.min(colCount - 1, Math.floor(pct * colCount));
};
/** A seat asked for that is not the one already chosen. The guard is what keeps a drag
* from re-committing the same level on every pointer move inside a seat. */
const choose = (seat: number) => {
if (seat !== selectedIndex) {
onChange(options[seat]);
}
};
const handlePointerDown = (event: PointerEvent<HTMLDivElement>) => {
const seat = seatAt(event);
if (seat === null) {
return;
}
event.currentTarget.setPointerCapture(event.pointerId);
setIsDragging(true);
setHoveredSeat(seat);
// Unguarded, unlike a move: a press on the level already chosen is the press that says
// "this one", which is how the seats answered before the track could be dragged.
onChange(options[seat]);
};
const handlePointerMove = (event: PointerEvent<HTMLDivElement>) => {
const seat = seatAt(event);
if (seat === null) {
return;
}
if (seat !== hoveredSeat) {
setHoveredSeat(seat);
}
if (isDragging) {
choose(seat);
}
};
const handlePointerUp = (event: PointerEvent<HTMLDivElement>) => {
if (!isDragging) {
return;
}
setIsDragging(false);
try {
event.currentTarget.releasePointerCapture(event.pointerId);
} catch {
// The pointer can already be gone; releasing twice is not an error worth surfacing.
}
};
/** The tint is the pointer's, so it goes when the pointer does — but not mid-drag, where
* a capture carrying the pointer off the control is still choosing a level. */
const handlePointerLeave = () => {
if (!isDragging) {
setHoveredSeat(null);
}
};
/** The arrows walk the seats, one level a press. The focus stays where it is: the
* control is one tab stop and the selection is what moves, so a level is never focused
* without being chosen. A value the list no longer holds walks to an end of it. */
const handleKeyDown = (event: KeyboardEvent<HTMLDivElement>) => {
const last = options.length - 1;
let next: number | null = null;
switch (event.key) {
case "ArrowLeft":
case "ArrowUp": {
next = Math.max(0, selectedIndex - 1);
break;
}
case "ArrowRight":
case "ArrowDown": {
next = Math.min(last, selectedIndex + 1);
break;
}
case "End": {
next = last;
break;
}
case "Home": {
next = 0;
break;
}
default: {
return;
}
}
event.preventDefault();
// The keyboard has no pointer, and a tint left behind by the mouse would be pointing
// at a seat the keys have already walked away from.
setHoveredSeat(null);
choose(next);
};
return (
<div
aria-label={ariaLabel}
aria-valuemax={colCount - 1}
aria-valuemin={0}
aria-valuenow={selectedIndex === -1 ? undefined : selectedIndex}
aria-valuetext={selectedIndex === -1 ? undefined : options[selectedIndex]}
className={cn(
"group relative cursor-ew-resize touch-none select-none outline-none",
className
)}
onKeyDown={handleKeyDown}
onPointerCancel={handlePointerUp}
onPointerDown={handlePointerDown}
onPointerLeave={handlePointerLeave}
onPointerMove={handlePointerMove}
onPointerUp={handlePointerUp}
role="slider"
tabIndex={0}
>
{/* The track, drawn behind the seats. The fill is the amount chosen and its leading
edge is the boundary of the chosen seat — the level is told once, as a length. */}
<div
aria-hidden="true"
className="absolute inset-x-0 top-0 h-9 overflow-hidden rounded-lg border border-border/50 bg-muted/20 group-focus-visible:ring-1 group-focus-visible:ring-ring"
>
<div
className={cn(
"absolute inset-y-0 left-0 overflow-hidden bg-accent",
isDragging
? "transition-none"
: "transition-[width] duration-150 ease-out motion-reduce:transition-none"
)}
style={{ width: `${filledPct}%` }}
>
{/* The drift, and only at the ceiling. Each mote's carrier spans the fill, so
`100%` of its travel is the fill's own width whatever the level is worth and
nothing is measured in pixels. `strength-mote` ships with the registry item
and is in globals.css for this page. */}
{isTop ? (
<span className="absolute inset-0 motion-reduce:hidden">
{MOTES.map((mote) => (
<span
className="absolute inset-0"
key={mote.id}
style={{
animation: `strength-mote ${mote.duration}s linear infinite`,
animationDelay: `${mote.delay}s`,
}}
>
<span
className="absolute left-0 size-1 -translate-x-1/2 -translate-y-1/2 rounded-full bg-foreground/60"
style={{ top: `${mote.top}%` }}
/>
</span>
))}
</span>
) : null}
</div>
{/* The playhead, standing on the fill's leading edge — the boundary of the level
chosen — and drawn only when there is a boundary to stand on. It is the bar the
duration slider carries, in the same place on the same track, because the two
controls stand in one column of one form. */}
{selectedIndex === -1 ? null : (
<div
className={cn(
"absolute inset-y-0",
isDragging
? "transition-none"
: "transition-[left] duration-150 ease-out motion-reduce:transition-none"
)}
style={{ left: playheadLeft(filledPct) }}
>
<div className="absolute top-1/2 left-1/2 h-6 w-1 -translate-x-1/2 -translate-y-1/2 rounded-full bg-foreground shadow-sm" />
</div>
)}
</div>
{/* The seats, and the names they carry. A seat is the pointer's map of the track and
the name under it is the level it stands for, so the two halves of the control
agree about which level is about to be chosen. The names are `aria-hidden`: the
level they spell is already the slider's `aria-valuetext`, and reading five names
out around it would name the levels twice. */}
<div
aria-hidden="true"
className="relative grid"
style={{ gridTemplateColumns: `repeat(${colCount}, minmax(0, 1fr))` }}
>
{options.map((option, index) => {
const isSelected = index === selectedIndex;
return (
<div
className="flex flex-col items-center gap-1 text-center"
key={option}
>
{/* The seat. The tint is the foreground token at 5% — the only thing that sits
on the fill and the empty track alike, because `muted` and `accent` are the
same lightness in light mode, so a `muted` hover would vanish on the part
of the track the selection already covers. */}
<span
className={cn(
"flex h-9 w-full items-center justify-center transition-colors",
seatRadius(index, colCount),
hoveredSeat === index && "bg-foreground/5"
)}
/>
<span
className={cn(
"w-full px-0.5 text-[10px] leading-[1.2] tracking-tight transition-colors",
isSelected
? "font-semibold text-foreground"
: "text-muted-foreground",
!isSelected && hoveredSeat === index && "text-foreground"
)}
>
{option}
</span>
</div>
);
})}
</div>
</div>
);
};