Styling lightning:combobox drop down to not hide inside modal and extend modal length?

Ugh, I finally found the culprit. The drop down was absolute positioned, but applying a higher z-index was not doing anything.

It turns out it was the overflow options of the slds-modal__container. They had to be 'unset' completely.

So doing a

.THIS .slds-modal__content{
    overflow: initial;
}

on my lightning component solved it.