processScrollDrag

pub fn processScrollDrag(
    self: *ScrollContainerWidget,
    sd: dvui.ScrollDragOptions,
) void {
    const rs = self.data().contentRectScale();
    var scrolly: f32 = 0;
    if (sd.mouse_pt.y <= rs.r.y and // want to scroll up
        sd.screen_rect.y < rs.r.y and // scrolling would show more of child
        self.si.viewport.y > 0) // can scroll up
    {
        scrolly = if (!self.seen_scroll_drag) -200 * dvui.secondsSinceLastFrame() else -5;
    }

    if (sd.mouse_pt.y >= (rs.r.y + rs.r.h) and
        (sd.screen_rect.y + sd.screen_rect.h) > (rs.r.y + rs.r.h) and
        self.si.viewport.y < self.si.scrollMax(.vertical))
    {
        scrolly = if (!self.seen_scroll_drag) 200 * dvui.secondsSinceLastFrame() else 5;
    }

    var scrollx: f32 = 0;
    if (sd.mouse_pt.x <= rs.r.x and // want to scroll left
        sd.screen_rect.x < rs.r.x and // scrolling would show more of child
        self.si.viewport.x > 0) // can scroll left
    {
        scrollx = if (!self.seen_scroll_drag) -200 * dvui.secondsSinceLastFrame() else -5;
    }

    if (sd.mouse_pt.x >= (rs.r.x + rs.r.w) and
        (sd.screen_rect.x + sd.screen_rect.w) > (rs.r.x + rs.r.w) and
        self.si.viewport.x < self.si.scrollMax(.horizontal))
    {
        scrollx = if (!self.seen_scroll_drag) 200 * dvui.secondsSinceLastFrame() else 5;
    }

    const before = self.si.viewport.topLeft();

    if (scrolly != 0 or scrollx != 0) {
        if (scrolly != 0) {
            self.si.scrollByOffset(.vertical, scrolly);
        }
        if (scrollx != 0) {
            self.si.scrollByOffset(.horizontal, scrollx);
        }

        dvui.refresh(null, @src(), self.data().id);

        // if we are scrolling, then we need a motion event next
        // frame so that the child widget can adjust selection
        dvui.currentWindow().inject_motion_event = true;
    }

    if (self.init_opts.user_scroll) |us| us.* = us.*.plus(self.si.viewport.topLeft().diff(before));

    self.seen_scroll_drag = true;
}