I don't know how, but the problem went away on its own.
Maybe it was due to a recent Opera update...

Unfortunately I was wrong: only the very first time after having a new profile it re-uses the window for new links. After that it still opens new a window for every external link...

Hmm, I just completely reinstalled Opera and then it works as expected (opens new links in same window).
Once I use my old profile folder it opens new windows on external links.
However, bringing back the blank profile fixes this.
So I can re-configure my profile and should have a working solution!

Yes, all those are Windows 10 machines.
I just renamed to profile folder (gives me a completely fresh start), but unfortunately the behavior is the same.

To add: This only happens on my office desktop computer. On my office notebook and on my home desktop Opera opens links from external applications in the existing window - as expected...

Daniel

When I click a link on a webpage in an opera tab then it either uses the same tab or opens another tab in the same window - good!

However, for any link that I click in another application (Thunderbird, HipChat, Sublime, etc) Opera always opens a new window to show the page. Instead I'd like to have a single Opera window only and any new tabs being opened in that window always.

I've not been able to find a setting for this. On my old computer I've been used for this to work as expected.

I'm running Windows 10, with the latest Opera (38.0.2220.41).

Thanks,
Daniel

I checked the history when it just happened, but I saw nothing that shouldn't be there.
I've now disabled all extensions and plugins. Let's see if it happens again...

Hi Joshl,

Thanks for the feedback.

It does then every now and then. Sometimes when Opera is already open and in the background and sometimes when I'm just browsing a web page. It doesn't need an action from me. I haven't found a way to trigger it myself. However, I don't know if it ever happened when I have no tabs open at all. So it could come from a page running JavaScript or so.

I am always using the latest stable release (31.0.1889.174 at the moment). I'm running Windows 7.
I have Adguard and Ghostery installed.

I notice the following weird behavior on my Windows machine:

Every now and then Opera opens a new window with just blank white content, then after a second or so the window closes again.
Is there a way to find out what triggers this?

Thanks!