Maybe helpful, maybe not
but I was facing a drawer with a q-scroll-area that was working and then it stopped working when I converted some content inside to use v-for, I was about to either just go with a div with overflow auto or look at calculating as mentioned above and then I added another div and it started working again. So I have a q-drawer with div.column, a couple div.col, then a div.col with the q-scroll-area using class=“fit” inside and at this point it was not working, then I added one more div (I used div-auto because of the design) and the scroll bar started working again.
I did also just notice that if I check safari I had native scroll bars appearing on one of the parent divs ( I have not looked into which) so out of curiosity I changed the drawer to behavior=“mobile” and the q-scroll-area stopped working again. Sig, so I suppose its back to calculating and a bit of extra height somewhere must be what made it work again.