Also, explicitly prevent drag events from firing when the context menu
opens. This will only be the case on macOS, since its context menu is
opened by Ctrl+mousedown. This replaces the prior exception preventing
drag events when Ctrl is held during mousedown.
Fixes#9018 and #9019
This allows dragging elements on the page and dropping them onto other
elements. This does not yet support dragging text.
The test added here is manual; the WPT tests rely heavily on WebDriver
actions.