After bug 1809084, the popup positioning code became more sound,
including reliably positioning once after opening.
This code was asynchronously reducing the size of the popup, sometimes
after popup positioning.
Which means that if the popup had to slide (due to having many one-off
buttons), and we then shrunk it, the popup would be mispositioned.
There's no reason the getBoundsWithoutFlushing call has to be async
afaict, so just size the popup up-front.
Differential Revision: https://phabricator.services.mozilla.com/D172928
0ae8f17683