Improvement for https://erripis.gr/Beirut/

index.js

/**
 * This prepares html
 * You can adjust your html directly and then delete this code block.
 */
//create container element which will have the exact width of its content (image)
const container = document.createElement('div')
//get your slider element
let slider = document.querySelector('.beforeAfterSlidebar')
//override position absolute (you need 1 image in dom flow for the container to set width properly)
slider.children[0].style.position = 'initial'
//append children to container element
let child
while (child = slider.firstChild) {
  container.append(child)
}
//append container to your slider
slider.append(container)

/**
 * Use this in your javascript
 */
//set your slider element
slider = container

//define event handler
function moveSlider(event) {
  const offsetX = event.offsetX !== undefined? event.offsetX : event.touches[0].clientX
  slider.querySelector('.topImage').style.width = `${offsetX}px`
}
//attach event listener
slider.addEventListener('mousemove', moveSlider)
slider.addEventListener('touchmove', moveSlider)