@leocg No no, I meant this one:

Today I turned on Opera and there was no way to go back to this style. I had to install older version (114.0.5282.159) to go back to it. The button you mentioned, offered only the modern styles.

@memuvd Thank you for this version! Some of the other 114.0.5282.x versions can't even run/initialize properly, but this one is good.

@leocg Hi, the button to switch the styles on the start page expectedly disappeared, but after clicking Opera GX Classic in Easy Setup, GX now has the same tab style as Opera >=100. Is the classic tab view gone for good?

@leocg In my case the good example is the Opera Forum page itself:

The CPU usage circles around 50-80%. When I close the forum pages, the CPU settles down.

I've noticed the same, eats up the CPU without any boundaries.

Hi, every time Opera GX receives an update, the sidebar reopens and shows itself. I don't want to use it and I hide it every time. Could you store this setting somewhere, so the sidebar doesn't reappear with each update?

@niechtoszlag Hey, just use latest iteration of Opera 95, I think it's the last good version of Opera. https://get.opera.com/ftp/pub/opera/desktop/
I'm using it and I don't see any issues, just turn off the autoupdate, so it doesn't break/update on its own.

@mat7thew Hi, I recommend you my solution from above, the latest Opera 95 version works very well and doesn't have the bugs introduced and not fixed in later versions.

@khellstre Hi, I found kind of a walkaround - using older Opera version, which I tried to describe here: https://forums.opera.com/topic/60852/why-did-they-remove-reattaching-browers-tabs/18?_=1686726458998
Maybe it will be helpful to you.

P.S. I'd be careful with commenting on mods here, I've seen a few bans that were shot, because people were telling their opinion, reasoning and sharing their frustrations regarding the worse and worse state of the browser. So you'd better not lean out in any way and say only good things about Opera. ;p Once I've even noticed the change of my post, because I wrote "EDIT:" instead of "Edit:", truly a law-breaker...

If for someone the "old" way of snapping windows and tabs to the tab bar is one of the crucial functionalities (like for me), I've found a solution for this problem. It seems that Opera 95 stable release is the best option and the only problem is forcibly maintain auto-update disabled. I've researched an auto-update subject a bit and I see it may be a bit of a problem.
Here's the short tutorial how to solve this issue:

1. Download and install latest release of Opera 95 from here: https://ftp.opera.com/pub/opera/desktop/95.0.4635.84/win/ - remember to select file that ends with Setup.exe or Setup_x64.exe, not Autoupdate one.
2. Go to installation location and remove all other versions' folders, leave only "95.0.4635.84" one.
3. Go inside this folder and remove all "opera_autoupdate" files. If you can't do this,
   3a. Open Task Manager, go to Users (for Win10, I don't remember for other systems), find "opera_autoupdate.exe" and end this process.
4. Remove scheduled startup Opera process, you can do this e.g. with CCleaner -> Tools -> Startup -> find and remove Opera assistant process from the list.
5. Edit the shortcut you're using to run the Opera by changing Target from launcher.exe to opera.exe (I'm not sure if it has any effect, it's just to prevent launcher from running anything else) and add "--disable-update" in the end.
   5a. If you're running Opera from the taskbar, unpin your current shortcut and drag'n'drop the one prepared in point 5. to replace it. This is important not to run the old shortcut, as it won't have the update disabled and it will spoil the whole operation by updating to the newest version.

So far Opera didn't update itself and this solution seems to be working properly - tabs are snapping like a charm, O-Menu icon is always on proper position and never disappears, window and tab management is back to normal.