When there is a absolute-positioned css grid, it's children aren't growing to fill it.
Minimal test case: https://codepen.io/Cactucs/pen/bjKGeX should show green page as in Chrome or Firefox. Instead it is red.
(Possible userland workaround is to make it into two elements as shown here: https://codepen.io/Cactucs/pen/GBGRya)
Tested in Browserstack on varios versions of safari (both desktop and mobile, including Safari TP) and Webkit Nightly.