Dragging the viewport scrollbar is accomplished by passing in a window
rather than an element.
Note that we can't just pass in the window's document.documentElement,
because coordinatesRelativeToScreen() would not give the correct result
for it. This is turn is because for a scrollable <div>, getBoundingClientRect()
returns the scroll frame's outer rect, but for the <html> element,
getBoundingClientRect() returns the root scroll frame's inner rect.
Differential Revision: https://phabricator.services.mozilla.com/D34258
--HG--
extra : moz-landing-system : lando
5a017eafd4