feat(headless): add headless mode

[myers]

Add headless mode entry point

This PR adds a new leva/headless entry point that enables using Leva's state management without automatically rendering the default UI panel. This is useful for building custom UIs in WebXR environments where you can use the DOM or React Three Fiber using 3d controls, or any scenario where you want Leva's hooks without the HTML controls.

What's New

New entry point: leva/headless

• Exports useControls wrapper that auto-injects headless: true
• No default Leva panel renders when using this import
• Full backward compatibility - existing code unchanged

Convenience hooks for building custom UIs:

• useLevaInputs() - Get array of all inputs with metadata
• useLevaTree() - Get folder tree structure
• useLevaInput(path) - Control specific input with full methods

Demo: Added leva-headless demo showing custom UI controls built from input metadata

Implementation Details

• Added headless parameter to HookSettings type
• Modified useRenderRoot call to skip when headless: true
• Exported parseArgs utility to reduce code duplication
• Headless wrapper reuses shared parsing logic (67 lines vs 87 lines of duplicated conditionals)

Example Usage

import { useControls, useLevaInputs } from 'leva/headless'

function MyCustomUI() {
  // Manages state without rendering default panel
  const values = useControls({
    speed: { value: 1, min: 0, max: 10 },
    color: '#ff0055',
    enabled: true,
  })

  // Get metadata to build your own UI
  const inputs = useLevaInputs()

  return (
    <div>
      {inputs.map(({ path, input }) => (
        <YourCustomControl key={path} path={path} />
      ))}
    </div>
  )
}

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Latest commit: 05d96e5

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Latest deployment of this branch, based on commit 05d96e5:

Sandbox	Source
leva-minimal	Configuration
leva-busy	Configuration
leva-scroll	Configuration
leva-advanced-panels	Configuration
leva-ui	Configuration
leva-theme	Configuration
leva-transient	Configuration
leva-plugin-plot	Configuration
leva-plugin-bezier	Configuration
leva-plugin-spring	Configuration
leva-plugin-dates	Configuration
leva-custom-plugin	Configuration

[myers]

@gsimone I'm glad to see some of my PR's getting merged. I would really love this one to get merged. Do you have any thoughts on it?

[gsimone]

@myers I'll take a proper look once I finished setting up tooling so we can start merging stuff more aggressively!

[myers]

@gsimone I have a fix for the global panel not being removed when switching between headless/headed mode. I wasn't thinking of allowing a switch like this for my use case. What I was shooting for was using this in WebXR with a uikit panel, where you can't use HTML.