A floating control reserves nothing; the content must

Aligning a control inside the Box that also holds the page puts it beside the content in the layout tree, not above it:

BoxWithConstraints(Modifier.fillMaxSize()) {
    Column(Modifier.fillMaxSize().verticalScroll(rememberScrollState()).padding(innerPadding)) {
        Spacer(Modifier.height(BACK_BUTTON_STRIP))   // the strip the button is drawn over
        TitleBlock()
        /* … */
    }