Hey I think i've figured this one out........... i got really frustrated because people have apparently come up with all manner of fixes for this kind of thing, and i tried them all and they didnt work for me i dont know why.. including javascript hacks, just to configure height??? come on.
the answer to your prayers here is using fixed positioning on the div you want to stay 100 %
this works with my example.. i made the pillar div, my semi transparent backdrop with no content inside as fixed.. i have to go test it on all the browsers and what not but i think this is the breakthrough and i hope this helps save somebody hours of clambering through awful forums with no clear directives
Cheers