I was about to post a topic regarding this 'issue' but found that it's already here.
Having a sidebar is great.. at the speed dial page!
Having a sidebar in every tab - not great!
Main reason being the white space on the left part of the screen where you click while browsing stuff. It's a 'default' white space easy to navigate to (swipe mouse to the left), to clear any selection on the page or activate a page. Right side of the browser is the scroll bar so you can't easily swipe and click there.
But when the sidebar is present in every window, this default functionality disappears so to speak.
Thus we have to disable the sidebar in order to have fast and smooth usage of the web.
Yet the side bar stuff itself is great, having whatsapp, bookmarks, Flow, and news right there with one click. But please have it in the speed dial only, just like it was there before.
Or have an option to choose where the sidebar should be active.
This needs to change back as it was before! The first adoption of the new sidebar was awesome. In user experience Opera has always been ahead of the others, at least the last 15 years I have been using this as a default browser (also use FF and Chrome daily, side by side with Opera).