I had also forgotten the other problem that solutions like position: fixed; bottom: 0; can bring, when the text is too long it collides with the footer, as below:
But I found a solution to replace min-height: -webkit-fill-available; and update my model, it is min-height: calc(calc(var(--vh, 1vh) * 100) - 40px);, now it works identically in Qt6 as in Qt5 and there is no overlap problem:
