If the current event loop task ( you may need to read up on the JS event loop ) was spawned by something like a click event then it is marked as a "user interaction". These tasks can do things like opening a popup window, start audio playback etc. whereas normal tasks are normally blocked from doing this.

It's a clever mechanism but malicious sites tend to just attach a click listener to the page and trigger everything when the user clicks for the first time.

On the other hand it can also be a pain to program around this if you need it. Say you have a webapp that allowed you render an image then display it in a popup. But the rendering is time consuming, so you do asynchronously in the background. The user clicks the button and you start the render. When the render is complete you try and open the popup, but your now in a different task where the popup is blocked. The best way to work around this is to display a modal dialog, saying that the render is complete. Then when the user hits "ok" you can use the task that the event created to display the popup. But it might confuse the user why they need to confirm the render is complete.

Couldn't you open the popup immediately while keeping a reference, do the proceeding, then just manipulate the reference to the window you just opened?