selenium设置窗口大小不准确

from selenium import webdriver

self.browser = webdriver.Firefox()
self.browser.set_window_size(1024, 768)
self.browser.get_window_size()    
    {u'height': 638, 'value': None, u'width': 1229}

测试时这样就过了：

inputbox = self.browser.find_element_by_id('id_new_item')
self.assertAlmostEqual(
    inputbox.location['x'] + inputbox.size['width']/2,
    # 512,
    self.browser.get_window_size().get("width")/2,
    delta=5
)