XR Controller events from multiple controllers in AR are sometimes mixed up / multitouch doesn't work

[hybridherbst]

Describe the bug

Seems on touchscreen devices (where input sources can change order when using multiple fingers), XR controller events are sometimes mixed up, which results in incorrect selectstart/selectend events.

To Reproduce

Steps to reproduce the behavior:

1. Go to https://fuchsia-brazen-thorium.glitch.me/ (https://glitch.com/edit/#!/fuchsia-brazen-thorium# to edit)
2. Open the page on an AR-capable mobile device
3. Enter AR
4. Try dragging one object - note that works
5. Try dragging two objects - note that doesn't work despite having two controllers, sometimes objects get "stuck" to fingers, sometimes nothing happens

Code

Same as on https://threejs.org/examples/?q=vr#webxr_vr_dragging

Live example

• https://fuchsia-brazen-thorium.glitch.me/ (for viewing on mobile)
• https://glitch.com/edit/#!/fuchsia-brazen-thorium# (for editing/remixing)

Expected behavior

Two fingers multitouch works because there's two controllers (and more fingers should work with more controllers).

Platform:

• Device: [Mobile]
• OS: [Android]
• Browser: [Chrome, Firefox]
• Three.js version: [dev]

[hybridherbst]

Verified again that this should be fixed by #23188, but seems there's some discussion around code clarity there...