I thought I saw how to do it.
.hs-shadow-container changes to hs-shadow-container.active when you open chat. If we had a similar functionality on the iframe, we could do something like this:
screen and (max-width: "YOUR BREAKPOINT GOES HERE") {
#hubspot-messages-iframe-container {
bottom : 60px !important;
}
#hubspot-messages-iframe-container.active {
bottom : 0 !important;
}
}