add axis to flip so spans w negative sign work correctly

src/core/span.rs

@@ -33,7 +33,7 @@ impl Widget for Span {
             }
             axis.bot_right.set(start);
             if self.dir.sign == Sign::Neg {
-                child_region.flip();
+                child_region.flip(self.dir.axis);
             }
             painter.widget_within(child, child_region);
         }