at max right scroll > hide right part not left part

[_onmouseclick]

I want do to a simple thing : I have a two scrolls website, and I want to hide the right part when the scroll is at the maximum on the right part. SO I used

document.onscroll = function() {
if (window.innerHeight + window.scrollY > document.body.offsetHeigh...

but the right part is hidden when the scroll is at his max on left part. How Can I do ?

the code pen with the precise problem : https://codepen.io/marie-fran-oise-talbot/pen/LKeqBE

thank youuuuu