When having an array of values and moving one of the thumbs by clicking on the slider, the index is not set properly before triggering the change event.
Steps to reproduce in the CodeSandbox:

• Open the Console Tab in the sandbox (or the browser dev tools)
• Click on the slider, causing one thumb to change
• The index will be -1 instead of 0 or 1 for the first click. All following indexes will have the index of the previous click

CodeSandbox

https://codesandbox.io/s/react-slider-bug-forked-9sckvq?file=/src/App.js