dropdown causes parent's vertical scrollbar to flicker

Hi,

I am using the dropdown to show a form and the first time showing it, the parent's vertical scrollbar appears and immediately disappears. Subsequent showing of the dropdown does not trigger the behaviour.

I have added a workaround in the css section as:
body {
overflow-y: hidden;
}

However, I am not sure if this is an ideal solution. I have specified the placement accordingly and the boundary as the parent's content pane, which is plenty big for it. Am I missing something?

Cheers,
Vlad