Q: Safari tabs unlocking from browser

I'm quite frustrated with El Crapitan. Whenever I have a safari open and three tabs open, if I hit one, it will unlock itself from the browser and make a separate window.  I cannot add it back to the original browser that was open. Often times, if I just hit it by accident once, it will create a new window. How to prevent that because it didn't happen with the previous OS (which is FAR superior to El crapitan on my macbook air.)