personal-website/resources/sys/wde/wde-scrollbar.js

class WdeScrollBar{
    /**
    * @param {HTMLElement} handler
    * @param {HTMLElement} content
    */
    constructor(handler, content){
        let nonNativeScroll = false
    
        handler.style.height = (content.clientHeight /content.scrollHeight)* handler.parentElement.clientHeight + 'px'
    
        let max = handler.parentElement.clientHeight - handler.clientHeight
        let yPosInit = 0
    
        handler.addEventListener('mousedown', (event) => {
            nonNativeScroll = true
            yPosInit = event.clientY - Number(handler.style.top.replace('px','' ))

            document.addEventListener('mousemove', drag);
            document.addEventListener('mouseup', stop)
        })

        content.addEventListener('scroll', (event) =>{
            if (!this.nonNativeScroll){
                let handlerPathLength = handler.parentElement.clientHeight - handler.clientHeight //TODO recalculate only on resize event
                let coefficient = (content.scrollHeight - content.clientHeight) /handlerPathLength
                handler.style.top = content.scrollTop/coefficient + 'px'
            }
        })

        function drag() {
            // console.log(event.clientY - yPosInit,  Number(handler.style.top.replace('px','' )))
            let pos = event.clientY - yPosInit
            let clampPos = Math.min(Math.max(pos, 0), max)
            handler.style.top = clampPos + "px";

            let handlerPathLength = handler.parentElement.clientHeight - handler.clientHeight //TODO recalculate only on resize event
            let coefficient = (content.scrollHeight - content.clientHeight) /handlerPathLength
            // console.log(clampPos, coefficient, content.clientHeight, clampPos* coefficient)

            content.scrollTop = clampPos* coefficient
        }
        function stop() {
            // console.log("stop")
            document.removeEventListener('mousemove', drag);
            document.removeEventListener('mouseup', stop)
            nonNativeScroll = false
        }
    }
}