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So, How Do Pop-under Windows Work?

Let's talk about two terrible ideas and how they conspired to annoy millions of Internet users.

onLoad / onUnload
blur / focus

These are JavaScript commands. onLoad and onUnload are what are called event handlers (they tell your browser what to do when something happens; in this case, when a browser window is either opened or closed). Blur and focus are used to bring a window to the foreground or send it to the background. Clever usage of the onLoad and blur commands allow advertisers to open a child browser window when you load the parent, and "blur" it into the background so that when you close the parent window, the ad sits on your screen like a lost puppy come home to the wrong house.

onUnload is the most scurrilous of all these commands, and it baffles me that any browser manufacturer would choose to adopt it. We'll talk more about its possible abuses later.

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