Fix issue with mixed border widths on Fabric (#11561)

* Borders should show if mixed widths but not colors
They were checking if the color was variable per side but assuming width wouldn't be.
Fixes #11549

* Change files

Author: chrisglein

change/react-native-windows-1d22a163-fe34-4bb8-84cd-16ca4ee138cb.json

@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "Borders should show if mixed widths but not colors They were checking if the color was variable per side but assuming width wouldn't be. Fixes #11549",
+  "packageName": "react-native-windows",
+  "email": "[email protected]",
+  "dependentChangeType": "patch"
+}

vnext/Microsoft.ReactNative/Fabric/Composition/CompositionViewComponentView.cpp

@@ -897,11 +897,11 @@ bool CompositionBaseComponentView::TryUpdateSpecialBorderLayers(
   // We only handle a single borderStyle for now
   auto borderStyle = borderMetrics.borderStyles.left;
 
-  if (borderMetrics.borderColors.isUniform()) {
-    if (!borderMetrics.borderWidths.left)
-      return false;
-    if (!facebook::react::isColorMeaningful(borderMetrics.borderColors.left))
-      return false;
+  bool hasMeaningfulColor =
+      !borderMetrics.borderColors.isUniform() || !facebook::react::isColorMeaningful(borderMetrics.borderColors.left);
+  bool hasMeaningfulWidth = !borderMetrics.borderWidths.isUniform() || (borderMetrics.borderWidths.left != 0);
+  if (!hasMeaningfulColor && !hasMeaningfulWidth) {
+    return false;
   }
 
   // Create the special border layers if they don't exist yet