switch to element defined span lens + better size fn

src/core/position/sized.rs

@@ -2,7 +2,8 @@ use crate::prelude::*;
 
 pub struct Sized {
     pub inner: WidgetId,
-    pub size: Vec2,
+    pub x: Option<Len>,
+    pub y: Option<Len>,
 }
 
 impl Widget for Sized {
@@ -10,7 +11,10 @@ impl Widget for Sized {
         painter.widget(&self.inner);
     }
 
-    fn desired_size(&mut self, _: &mut SizeCtx) -> UiVec2 {
-        UiVec2::abs(self.size)
+    fn desired_size(&mut self, ctx: &mut SizeCtx) -> Size {
+        Size {
+            x: self.x.unwrap_or_else(|| ctx.size(&self.inner).x),
+            y: self.y.unwrap_or_else(|| ctx.size(&self.inner).y),
+        }
     }
 }