Web: value={0} with a negative minimumValue renders the thumb at the minimum #828

Environment

• @react-native-community/slider: 5.2.1 (also present on main at the time of writing)
• Platform: web (react-native-web 0.21.2)
• react: 19.2.3 · react-native: 0.86.3 · Expo SDK 57
• New architecture: yes (Expo newArchEnabled is unconditional in SDK 57) — but this is a web-only code path, so the architecture is not relevant

Description

On web, a slider rendered with value={0} and a negative minimumValue paints its thumb at the minimum instead of at zero.

The value itself is fine — onValueChange reports correctly, and every number the app derives from the slider is right. Only the thumb position is wrong, and it stays wrong until the user drags it, at which point it starts behaving. So the failure looks like a rendering quirk rather than a state bug, which makes it easy to dismiss.

It is quite easy to hit in practice: any bipolar control that rests at zero does. Mine is a turntable-style pitch fader (-8 … +8, resting at 0), which means it was wrong on every load.

Reproducible Demo

import Slider from '@react-native-community/slider';

// Thumb renders hard left. Expected: centred.
<Slider minimumValue={-8} maximumValue={8} value={0} />

// Both of these are fine, which isolates it to the value being exactly zero
// against a negative minimum:
<Slider minimumValue={-8} maximumValue={8} value={0.1} />
<Slider minimumValue={0}  maximumValue={16} value={8} />

Cause

Two things combine, both in package/src/RNCSliderNativeComponent.web.tsx.

1. The initial state is seeded with || (L81):

const [value, setValue] = React.useState(initialValue || minimumValue);

0 is falsy, so value={0} seeds the internal state at minimumValue rather than at 0.

This cannot be fixed with ?? alone, because value is already defaulted at the destructure (L55):

value: initialValue = 0,

so by the time that line runs, "no value prop" and value={0} are indistinguishable — both are 0.

2. The resync effect cannot recover it (L163-L171):

const lastInitialValue = React.useRef<number>(0);
// ...
if (initialValue !== lastInitialValue.current) {
  lastInitialValue.current = initialValue;
  const newValue = updateValue(initialValue);
  animationValues.val.setValue(newValue);
}

The ref starts at 0, so for initialValue === 0 the guard evaluates 0 !== 0 — false. The effect never runs and the wrong initial state is never corrected.

I appreciate the guard is deliberate; the comment above it explains that it exists because initialValue gets defaulted to 0 by Slider, and without it the value resets constantly when the slider is uncontrolled. Any fix wants to keep that property.

Suggested fix

Distinguish "no value prop" from value={0} by not defaulting at the destructure, and track the same distinction in the ref:

value: initialValue,                                    // no ` = 0`
// ...
const [value, setValue] = React.useState(initialValue ?? minimumValue);
const lastInitialValue = React.useRef<number | undefined>(initialValue);
// ...
if (initialValue !== undefined && initialValue !== lastInitialValue.current) {
  lastInitialValue.current = initialValue;
  const newValue = updateValue(initialValue);
  animationValues.val.setValue(newValue);
}

That keeps the uncontrolled case from being reset (the effect still does nothing when no value is passed) while letting an explicit value={0} through.

I have not sent a PR since the change touches the uncontrolled path and you will have a clearer view than I do of what that should do; happy to open one if the approach looks right.

Workaround

For anyone hitting this before it is fixed: shift the domain so zero is not the resting value, and convert at the boundary.

// fader units 0 … 2R, presented as -R … +R
<Slider
  minimumValue={0}
  maximumValue={range * 2}
  value={pitch + range}
  onValueChange={(v) => setPitch(v - range)}
/>

The only value that still lands on 0 is the control fully down, where minimumValue is also 0, so the falsy branch produces the correct result anyway.