The majority of those threads come from people “me too”-ing that they are also experiencing the issue and would like a setting to make it work as they desire. I can certainly agree that this should be a feature request and not a bug report. In Chromium’s bug tracking, I found Issue 10511 and Issue 5348 each requesting this ability to have popups open as tabs many years ago.
In Firefox settings, I believe the “Open new windows in a new tab instead” is responsible for operating as desired. However, in Firefox and even Firefox for Android, the setting works quite reliably. The resulting behavior can differ depending on the situation. You might find yourself in a place where it matters if a click was user-initiated or automated. Now Javascript for popups is hardly an ideal world. However, I found a number of others that also agreed that it was a feature they would like to see introduced and some even listed it as a reason for why they would go back to using Firefox as their primary browser. Unfortunately, there does not appear to be a solution that works in Chrome for all cases. It’s a very minor issue but I hadn’t looked into it for a while to see if there was some sort of solution out there. Google Chrome, on the other hand, opens each popup in its own window.
In Firefox and Internet Explorer, the feature works great as it opens a new tab for each thread and everything works as expected. I further modified it to be able to open all of those unread threads in a single click. I modified a phpBB installation to keep track and indicate unread posts.
I spent some time this weekend looking into a past feature request and minor annoyance that has to do with Chrome.