In the video you can see the correct way how both the offcanvas and the page should behave. Explanation:
Both the offcanvas and the body have their own scroll generated by the browser according to the height of their content, therefore both are scrollable independently. And human intuition makes that if you open the menu and it is only 50% of the width of the screen you have to act on it, and if the scroll has finished (up or down) the browser captures the mouse in the next element that has scroll which would be the page, for example if you are going down in the offcanvas and there is no more content down but you keep scrolling the browser captures it for the page, and if you want to go back it will continue to be captured, just stop and go back in the offcanvas and you will see that it is working on the offcanvas again.
Can it be changed? Of course, but it would be unnavigable when the links of the menu elements are #ids of sections in ONEPAGES.
If you don't use onepage and want to change the behavior you can try:
Code:
.offcanvas-active .body-wrapper{max-height:100vh;overflow:hidden;}
This will remove the scrolling of the page when offcanvas is open.