Remove background:transparent setting for popups.
Not sure why that was there; it seems like the opposite of what we want.
Stop automatically applying overflow:auto CSS to dropdown root node.

The dropdown should handle where the overflow:auto goes because it could
be on a descendant node rather than the root node,
thanks to Scrollable, LinearLayout, flex-box CSS, etc.

Also fixed the popup doc about how the popup indicates it was executed/canceled.
various migration notes, mainly related to changes in 0.4.