REGRESSION: Select listbox dispatches a change event on scroll.
wkrev.com/143560 fixed mouse events on scrollbars, and broke an
HTMLSelectElement assumption that mouseup event always happens on an
<option> element. So, listBoxOnChange() was called unexpectedly.
This patch avoids this problem by checking
m_lastOnChangeSelection.isEmpty(), which means we didn't start click
or drag in this <select> element.