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);
         }

src/layout/pos.rs

@@ -87,13 +87,14 @@ impl UiVec2 {
         }
     }
 
-    pub fn flip(&mut self) {
-        self.rel = 1.0 - self.rel;
-        self.abs = -self.abs;
+    /// reflection about an axis
+    pub fn flip(&mut self, axis: Axis) {
+        *self.rel.axis_mut(axis) = 1.0 - self.rel.axis(axis);
+        *self.abs.axis_mut(axis) = -self.abs.axis(axis);
     }
 
-    pub fn flipped(mut self) -> Self {
-        self.flip();
+    pub fn flipped(mut self, axis: Axis) -> Self {
+        self.flip(axis);
         self
     }
 
@@ -227,10 +228,13 @@ impl UiRegion {
         }
     }
 
-    pub fn flip(&mut self) {
-        self.top_left.flip();
-        self.bot_right.flip();
-        std::mem::swap(&mut self.top_left, &mut self.bot_right);
+    pub fn flip(&mut self, axis: Axis) {
+        self.top_left.flip(axis);
+        self.bot_right.flip(axis);
+        let tl = self.top_left.axis_mut(axis);
+        let br = self.bot_right.axis_mut(axis);
+        std::mem::swap(tl.rel, br.rel);
+        std::mem::swap(tl.abs, br.abs);
     }
 
     pub fn shift(&mut self, offset: impl Into<UiVec2>) {