Reason: The UI (Chromium) does not allow vertical screen division.

This is a pretty big problem, but there is a solution:

Currently, all of the new opera://* pages are coded in HTML as web pages, where in the past these were built into the UI. It could be solved by instead adding javascript on each page which created HTML panels, although this would be a very messy fix.

This is a fair option that might be useful when browsing high-gif intensity sites like tumblr.
If we're talking about animated image formats, it would be nice to see some full support for APNG.

If I'm in a forum sub-topic, like "General Topics > Forum Feedback", If I press the "new discussion" button it doesn't auto-select the sub-topic I was just in. Is that intended behaviour?

Have you checked to see if your browser is using the options:

• Predict network actions to improve page load performance
• Use a prediction service to help complete searches and URLs typed in the address bar

These might be referring to google services, but I'm not sure.

Panels were an amazing feature of classic opera. It would be great to see panels make a return to the new opera. If it makes any difference, here are the panels that I would like to see, prioritised top to bottom:

• Bookmarks
• Downloads
• History
• Info

I did also use the notes panel quite often, but I now use Microsoft onenote instead.