Grid shows 500px width in Vaadin 8

I am trying to have a grid take up the full width of a defined-size Window.
If I leave the grid with size undefined, it displays a width of exactly 500px. Where does this magic number come from?
(Setting a width of 100% displays an actual width of 0)

Thanks,
Reuben

Setting a width of 100% displays an actual width of 0

What you mean by that? Width of 0px would mean that Grid does not show at all? Or do you mean that getWidth() returns 0, but not the actual width?

does not display at all.