COPY & PASTE solution
Here is an easy drop-in solution based on user11990065's answer to set a css variable --scrollbar-width and keep it updated on resizes.
It also gets calculated on DOMContentLoaded and load events so that you don't have to worry about size changes during the initial rendering phase.
You can just copy and paste it to your code as it is vanilla JS (or wrap it in a 'script' tag and paste it directly into your HTML code:
function _calculateScrollbarWidth() {
document.documentElement.style.setProperty('--scrollbar-width', (window.innerWidth - document.documentElement.clientWidth) + "px");
}
// recalculate on resize
window.addEventListener('resize', _calculateScrollbarWidth, false);
// recalculate on dom load
document.addEventListener('DOMContentLoaded', _calculateScrollbarWidth, false);
// recalculate on load (assets loaded as well)
window.addEventListener('load', _calculateScrollbarWidth);
If you have dynamic height changes in your page that might show / hide the scrollbar, you might want to look into Detect Document Height Change with which you can trigger the recalculation also on height changes.
As the value is calculated with JS and set to a fixed value you can use it in calc operations in your CSS, like so:
.full-width {
width: calc(100vw - var(--scrollbar-width));
}
This will give .full-width exactly the available width.
width: calc(100vw - scrollbarWidth)
where 'scrollbarWidth' is some fixed value like 15px – Scissor