switch to element defined span lens + better size fn

src/core/position/offset.rs

@@ -11,7 +11,7 @@ impl Widget for Offset {
         painter.widget_within(&self.inner, region);
     }
 
-    fn desired_size(&mut self, ctx: &mut SizeCtx) -> UiVec2 {
+    fn desired_size(&mut self, ctx: &mut SizeCtx) -> Size {
         ctx.size(&self.inner)
     }
 }