MenuButton and SplitMenuButton are not consistent in supporting the press-drag-release sequence

MenuButton and SplitMenuButton must be consistent in when they open the drop-down. Say when I press the mouse on the arrow of the SplitMenuButton, I expect the popup to appear. However, it appears only when I release the mouse. Whereas the menu button supports press-drag-release gesture in accessing the menu items. I feel it would be nice if split menu button is also made consistent with menu button in supporting the press-drag-release sequence.

You can make out the difference with the attached code.

I reproduced it on WinXP - Soma-graphics-scrum-b860 - 6u18.