WebElement resizeHandle = browser.findElement(By.id("myForm:popupResizerSE"));
Actions builder = new Actions(browser);
final Action dragAndDrop = builder.dragAndDropBy(resizeHandle, 40, 40).build();
dragAndDrop.perform();
Assert.assertEquals("Container width", "340px", container.getCssValue("width"));
Assert.assertEquals("Shadow width", "340px", shadow.getCssValue("width"));
Assert.assertEquals("Container height", "240px", container.getCssValue("height"));
Assert.assertEquals("Shadow height", "240px", shadow.getCssValue("height"));