Resolve deferred size comparisons before allocating span slots

src/widget/position/stack.rs

@@ -8,6 +8,16 @@ pub struct Stack {
 }
 
 impl Widget for Stack {
+    fn size_request(&self, requests: &mut SizeRequests, axis: Axis) -> Option<RequestedLen> {
+        match self.size {
+            StackSize::Default => Some(LayoutLen::LEFTOVER.into()),
+            StackSize::Child(i) => match self.children.get(i) {
+                Some(child) => requests.widget(child, axis),
+                None => Some(LayoutLen::LEFTOVER.into()),
+            },
+        }
+    }
+
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let sizing = match self.size {
             StackSize::Default => None,