Thanks for the replies. In the end I started using Chrome for a while and confirmed that the problem does, indeed, also affect that browser. Then, a quick search revealed that the problem is fairly well documented!

http://tinyurl.com/q3x8erj

It seems that the transition between the two GPUs on my laptop (nVidia for docked, Intel for undocked) is suspected to be the culprit. A work around is to go to opera://flags and set "Enable GPU rasterization." to Disabled.

Hmmm, it is rather disappointing not even to have found confirmation of this happening on others' computers. Does anyone else here run a Thinkpad which they dock and undock and not suffer from the problem?

Is there any way that I can gather some information that might help diagnose this? Perhaps there is a debug mode or some log files that I could grab?

Since I have found exactly the same issue with Vivaldi, I have made a similar post over there:
https://vivaldi.net/en-US/forum/vivaldi-browser/3960-opera-unresponsive-after-docking-undocking#30032

I'm running Opera on a Thinkpad (T530) with native resolution 1920x1080, and I dock with a setup including a 1920x1200 resolution monitor. I do not know whether the resolutions are significant here, but I list them anyway because I feel as though it might play some part. I'm running Win 7 Pro.

Sometimes, after docking/undocking, Opera becomes unresponsive, which I usually notice because it does not appear when I click on the taskbar icon. Sometimes I can kill it by right clicking and selecting close, twice, other times I have to use Process Explorer to kill the root process. Luckily, 95% of the time tabs are remembered, but occasionally the "kill" must have been more drastic and Opera has reopened without any tabs.

This problem has existed for many versions, and it seemed like the sort of thing which would be fixed without me needing to make a specific post, but the fact that it has not leads me to believe that perhaps it is not as common as I suspected. Recently, I have been experimenting with the Vivaldi browser and it has exactly the same problem, suggesting that it is an issue with Chromium. I have not tried using Chrome extensively, but occasionally I have it open and I have never noticed it suffering from this problem.

Has anyone else who uses a laptop which is frequently docked/undocked suffered from this problem? How about anyone who does not dock, but who has a setup which changes resolution for some reason? Other Thinkpad owners?

In order not to make "repeat requests", I merely submit my vote for the return of tab stacking here!

I agree - I still think that Opera Mail holds its own. For me, the rapid searching and grouping of mails across multiple accounts are the highlights. It does need a bit of work, but I still recommend it to people because there is barely any competition.

Oh, cool, thanks! I don't have time for development releases, so patience it is. Thanks for the heads up.

Sorry, I should have said. I'm on 28, although that update happened only this morning from 27. The problem has been happening this week, although I just did something which nearly always results in one "missed tab" (an add-on that blasts open around 20 different search tabs simultaneously) and every tab succeeded. Perhaps they just fixed it in 28! I'll report back.

So, my question is, can I hack the configuration files to force the old behaviour?!