In web browsers, a popup-blocker is browser code that attempts to prevent annoying or disruptive unsolicited popup windows from polluting the display area.
Most browsers implement popup blockers by attempting to correlate user actions to window creation activities in script. If a window is opened by code that is responding to a mouse click event, the popup blocker allows the window to be shown.
If a window is opened in browser script that is not directly linked to a mouse click or other user input action, the popup blocker will probably prevent the window from being shown on the premise that the user didn't do anything to warrant a response from the app. Opening a window in a timer event handler, for example, will usually run afoul of popup blockers because there is little or no association between the timer event handler that opens the window and the user mouse click handler that created the timer event.
The definition of "proximity" of code to a user event varies by browser implementation. For example, Internet Explorer 6 would only allow code within a mouse click event handler or within about 2 nested call frames of a mouse click handler to open a popup window. In Internet Explorer 7, the nested function call tolerance was much greater, so long as the call chain originated in a mouse click event.
