By the way, i have tested on both 2.0 and 2.1 on same server and it doesn't happen on 2.0 but 2.1 has this issue. So surely not device or server related issue but issue on XF 2.1
Well it could still be device, browser or server specific in relation to how XF is implementing it. I have no issue on my iPad, iPhone or Fire, the menu slides as expected.