Como alternar para uma nova janela pop-up usando selenium / java

Se você deseja manipular a janela filho, use as alças em selênio, consulte o código abaixo:

String parentWindowHandle = driver.getWindowHandle(); // get the current window handle

//Perform action on your parent window 
//Perform clcik() action on your parent window that opens a new window    

for (String winHandle : driver.getWindowHandles()) {

         if(!winHandle.equals(parentWindowHandle))
         {
            driver.switchTo().window(winHandle); // Here yor switching control to child window so that you can perform action on child window
            System.out.println("Title of the new window: " +
            driver.getTitle());
            //code to do something on new window
            System.out.println("Closing the new window...");
            driver.close();
         }

   }   

driver.switchTo().window(parentWindowHandle);
System.out.println("Parent window URL: " + driver.getCurrentUrl());