Change Proposal: Increment/Decrement Spinner Buttons for `NumberInput`

[zoharma]

What is being proposed?

Add optional up/down spinner buttons to NumberInput so users can nudge a numeric value by a configurable step, instead of only being able to type a value directly.

Why is this needed?

NumberInput currently only accepts typed input. For small, discrete adjustments (e.g. incrementing an integer count), typing is slower and more error-prone than a click-to-step control, which is a common affordance for numeric fields. There was previously no way to adjust the value without full keyboard entry.

What will change?

• Behaviour: NumberInput can now render a vertical up/down ButtonGroup inside the field as an end adornment. Clicking increases/decreases the current value by step, clamped to minValue/maxValue. Buttons disable automatically at the limits.

• New props:

  • step?: number: the increment/decrement amount, defaults to 1.
  • spinnerButtons?: "always" | "onHover" | "never": controls spinner visibility. Defaults to "onHover" for all number modes except "scientific", which defaults to "never" (since stepping isn't a natural operation on scientific-notation values).
  • No new component: this extends the existing NumberInput.

Interface changes (if any)

Additive/optional only — existing usages of NumberInput are unaffected.

<NumberInput
  label="Quantity"
  numberMode="natural"
  step={1}
  spinnerButtons="always" // "always" | "onHover" | "never"
/>

Breaking change?

• Yes
• No

Next steps

A maintainer will review this issue.
If accepted, it will be marked as accepted and a PR may then be opened.

[zoharma]

@VictoriaBeilsten-Edmands I made the PR #271 after seeing that the arrows were missing in a NumberInput in Atlas/Waffle with this control. But then thought it probably needs to go through the proper routes. So here's an issue-proposal for it.