switch to element defined span lens + better size fn

src/core/position/stack.rs

@@ -24,9 +24,9 @@ impl Widget for Stack {
         }
     }
 
-    fn desired_size(&mut self, ctx: &mut SizeCtx) -> UiVec2 {
+    fn desired_size(&mut self, ctx: &mut SizeCtx) -> Size {
         match self.size {
-            StackSize::Full => UiVec2::MAX_SIZE,
+            StackSize::Default => Size::default(),
             StackSize::Child(i) => ctx.size(&self.children[i]),
         }
     }
@@ -35,7 +35,7 @@ impl Widget for Stack {
 #[derive(Default, Debug)]
 pub enum StackSize {
     #[default]
-    Full,
+    Default,
     Child(usize),
 }