@burnout426 @howbizarre I have this issue as well.
If it helps, it's because Opera is reporting the wrong viewport width/height on initial page load with dev tools open. It's reporting the full size of the window rather than minus the dev tools. So if you follow links with dev tools open, the next page you load will be wrong.
If you add this to a page, you can see what it's doing:
function logViewportSize() {
console.log('Width:', window.innerWidth);
console.log('Height:', window.innerHeight);
}
logViewportSize();
window.addEventListener('resize', logViewportSize);
It's only on initial load of a new page. Any resizing, either of the window or dev tools, it will then report the correct width. Initial opening of dev tools also corrects the viewport numbers.