I’ve been having this issue where if I join two tabs by doubleclicking on one of tab titles, the window always goes max size. I can resize it but the issue always occurs when merging tabs. I did a little bit of troubleshooting and it appears that it occurs when using the Narrow Tabs option. If I disable that, it works fine.
Interesting. I have just tested it here briefly on my Mavericks machine and I don’t see this problem with narrow tabs enabled (I used keyboard shortcut to toggle it on/off).
Also I thought your clicks could be somehow interpreted as double-clicking the window area behind tabs. But on my machine clicking tabs background does window miniaturization, not zoom.
Any chance you are running some 3rd party system-wide utility for window management or zoom tweak.
Ah, now I see you included OS version in the title. Let me try it under latest Yosemite.
Turns out it still happens with Narrow Tabs unchecked as well, just not as often. I’m not using any type of window management app that I know of. I think the closest I was was Hyperdock but I disabled it thinking maybe that may have caused something like this. Still happens.