Javascript Robotframework/Selenium2Library：为什么拖放不适用于可拖动的元素？

如果安装了python v2.7、robotframework v2.8.7和Selenium2Library v1.6.0，则可以运行以下测试用例来了解问题，例如使用：pybot-L TRACE

*** Settings ***
Library           Selenium2Library

*** Variables ***

*** Test Cases ***
DragDemo
    Open Browser    http://www.w3schools.com/html/html5_draganddrop.asp    firefox    
    # NO VISIBLE ACTION HERE, BUT COMMANDS TAKE SOME MINUTES TO EXECUTE WITHOUT FAILURE
    Wait Until Page Contains Element    //img[@id='drag1']
    Drag And Drop   //img[@id='drag1']  //div[@id='div2']
    Drag And Drop By Offset     //img[@id='drag1']  50  0
    Drag And Drop By Offset     //img[@draggable='true']    100     0
    Drag And Drop By Offset     //img[@alt='W3Schools']    10     -10
    Mouse Down  //img[@id='drag1']
    Mouse Over  //div[@id='div2']
    Mouse Up    //div[@id='div2']
    # DRAG AND DROP ACTION HERE
    Drag And Drop By Offset     //span[contains(text(),'Drag and Drop')]    500     -500
    Sleep   5

*** Keywords ***    

该测试导航到公共网站，并尝试以各种方式拖放可拖动的元素。正如内联注释中提到的，这些命令执行时没有失败，但没有明显的效果。然而，最后一个拖放命令是在不可拖动的元素上执行的，奇怪的是，标记了一些操作文本。测试日志显示POST在任何一种情况下都会执行。为什么其他DnD命令不能按预期工作