Can't close tabs on browsers using middle-click unless the browser has focus

I have just recently acquired a Mac Pro 3.1 and have been a strictly Windows/Linux guy my whole life. But I'm really liking Mac and starting to get used to it.

I imagine that I'll be on this stackexchange a bit more in the near future, and so I had one basic question to get started.

Basically, as the title says, when my browser doesn't have focus e.g. chrome/safari in the background while using iMessage, I can't move my mouse over a tab and middle click to close a tab, which I was used to being able to do in windows. The middle click does work to close tabs when the browser has focus. Obviously, this is a very minor issue, and I was just wondering as to whether or not this is supposed to happen.

Another little issue is that I can't middle click and drag or middle click then move mouse to scroll on pages and I was wondering if there is another function for that or if it's my mouse (Asus Reign of Gaming mouse from my PC).

Any help/advice is appreciated! Thanks!

Unfortunately, I'm afraid you're out of luck here. OSX never gives focus to a window unless you left-click on it or Cmd-Tab to it. There were some older solutions that could provide focus-follows-mouse, though they're not supported in the newest versions of OSX.

Sadly, middle-clicking non-focused Chrome tabs used to close them. But I believe this feature was lost once Mavericks came out. You can middle-click on some other apps (like Terminal), and they'll respond appropriately. But no browser seems to do so.

As for your question about using middle-click-and-drag to scroll, that's something I dearly miss from Windows, but I haven't found a free way to get it on Chrome in OSX (it works in Firefox, though). Smart Scroll is an app that offers "Vector Scroll", which works like the feature we want. But it costs $15.