Yes, I have Chrome Extensions installed. Actually I have the https://everywhere extension installed in Opera Stable version, so I tried to install it in the beta, but no go so far.
Using stable opera I still have the problem if the builtin adblock is enabled, but not if the adblock is off - the sites stay visible for several seconds longer today, but disappear. Using opera beta 60.0.3255.8 there is no problem at all.
You can go into opera://settings/handlers, delete mail.google.com, restart Opera, go back to mail.google.com, view some messages (if needed) to get Gmail to ask you to associated mailto again and allow it to see if that helps.
www.opera.com
Opera homepage > click Blogs on the right > click Product blogs > click 'Desktop team / English' > click any blog post > the link to the full changelog will be at the bottom
When you select the specific product blog, I agree there should be a link to the overall changelog collection (which is missing), but the url for it is: https://blogs.opera.com/desktop/changelog-for-45/
Try with Steven Black Hosts, in AdGuard there are some lists to subscribe, check for your navigation countries, and may be need it to manual block some ads.
Full text index, the best Opera Presto feature. (Almost) No need to bookmark anything but just remember what the page was about or its content.
But really, with information and pages changing too often, there always be a chance that bookmarked pages are obsolete after a little time. Also you can find most pages with search engines.