You can't merge now - the flag is gone.
I've been using vertical tabs for years. The Vertical Tabs extension in Opera. The extension sidebar has to be visible all the time with my browser tabs.
The issue is not the merging though, but the pinned sidebar pane (with the extension content) occupying so much space vertically, from the top to bottom. The main browser pane along with the URL and favorities bars are much shorter now. It just doesn't make much sense. Not useful.
There should be an option for this behavior.

Warning: Opera 65 doesn't display PDF files if this extension is enabled! It's confirmed.
The author is not responding.

@leocg OK, I know, but why no option to have the bar below, like previously? I mean not to take the full height? Damn you Opera developers! Now the address bar +extension icons and the bookmarks bar are shorter. Why???

Chrome does it better!

1. More copmact (less space among tabs).
2. You can see if a tab group is collapsed or expanded (right-side arrow icon on a tab-group bar. Critical! Why in Opera there is the number instead, and we can't tell the collapse-status???
3. We can clearly see which tabs are in a group - there is a vertical line on the left side. Opera has shaded color below the group - not that clear, combined with 4):
4. The active tab color is hard to distinguish from the shaded colors of in-group tabs.
5. The wonderful group buttons in the favourites bar, which can jump to other Chrome windows.
6. The last "New tab" tab is just "+", Opera "New tab" text looks like a tab name - misclickable.

You can't merge now - the flag is gone.
I've been using vertical tabs for years. The Vertical Tabs extension in Opera. The extension sidebar has to be visible all the time with my browser tabs.
The issue is not the merging though, but the pinned sidebar pane (with the extension content) occupying so much space vertically, from the top to bottom. The main browser pane along with the URL and favorities bars are much shorter now. It just doesn't make much sense. Not useful.
There should be an option for this behavior.

@leocg OK, I know, but why no option to have the bar below, like previously? I mean not to take the full height? Damn you Opera developers! Now the address bar +extension icons and the bookmarks bar are shorter. Why???

@gmiazga Knock-knock... hello... Opera developers... from November to January, from v65 to 66... it's been a long time... we the Translator extension users still can't see PDF files! Are you working on this issue?

Do Opery nieaktualizowane v.0.1.2 i od kilku miesięcy nie działa...
Wersja dla Chrome 0.1.5 działa z Operą ok.

Warning: Opera 65 doesn't display PDF files if this extension is enabled! It's confirmed.
The author is not responding.