How do you handle multiple windows and tabs?

Each top-level browsing context has a window handle. A click that opens a new tab changes the set of handles. The driver stays on the original window until you switch explicitly.

String original = driver.getWindowHandle();

driver.findElement(By.linkText("Open invoice")).click();

WebDriverWait wait = new WebDriverWait(driver, Duration.ofSeconds(10));
wait.until(ExpectedConditions.numberOfWindowsToBe(2));

for (String handle : driver.getWindowHandles()) {
    if (!handle.equals(original)) {
        driver.switchTo().window(handle);
        break;
    }
}

Assert.assertTrue(driver.getTitle().contains("Invoice"));
driver.close();
driver.switchTo().window(original);

• getWindowHandle returns the current window; getWindowHandles returns all open handles
• Handle strings are opaque and their order is not guaranteed
• switchTo().newWindow(WindowType.TAB) opens a new tab directly in Selenium 4
• close closes the current window, while quit ends the whole session
• A closed handle becomes invalid; switching to it throws
• Window state such as cookies and local storage is shared within the same browser session

Do not assume the second handle in the set is the new one. Either diff against the handles captured before the action or use the window title or URL to identify the target. If the action can open zero or several windows, encode that expectation instead of silently taking the first handle.