In IE11 at least, the default user setting is to 'Let Internet Explorer decide how popups should open' (Off-hand, I'm not sure what the default was in previous versions). Some context around the IE behaviour with this default setting can be found here: https://superuser.com/questions/713743/what-rules-apply-when-ie9-decides-how-to-open-popups
Per others' answers/comments here, generally speaking, we as web programmers have little control over how popups are opened. However, provided users have not changed their default selection (in IE11 at least!), you can approximate the behaviour you want through appropriate invocation of the window.open() method. Specifically, if you pass less than three arguments to the method, IE will generally open a new tab, while passing the specs
(third) argument (even if with a null or undefined value <- this caught me out initially!) results in IE popping a separate window.
If you're happy to live with a solution that isn't guaranteed but works a fair amount of the time, this may help. The 'tyranny of the default' works in our favour in this instance at least!
Chrome at least is not quite so picky about the specs
argument - if null or undefined, it will pop the tab instead of the window under default configuration.
_blank
links in new windows, but new tabs. What version of IE did you test in? – Šime Vidas