Created attachment 324819 [details]
Test to reproduce the issue
This is easily reproducible if you combine "height: -webkit-fill-avilable" with "box-sizing: border-box"
and the element has some border and/or padding.
Attached example to reproduce the issue. And current output, the expected result would be that both cyan boxes
(the one with "content-box" and the one with "border-box") take the whole height of their wrapper.
BTW this is working fine in Chromium/Blink.
Created attachment 324820 [details]
This was fixed past year in Blink: