Not discarding base widget anymore,using BoxDecoration instead

Author: MdSaifAliMolla

lib/widgets/katex.dart

@@ -131,10 +131,11 @@ class _KatexSpan extends StatelessWidget {
           : currentColor;
       final double borderWidth = borderStyle.widthEm * em;
 
-      return Container(
+      widget = Container(
         constraints: const BoxConstraints(minWidth: double.infinity),
-        height: borderWidth,
-        color: borderColor,
+        decoration: BoxDecoration(
+          border: Border(bottom: BorderSide(color: borderColor, width: borderWidth))),
+        child: widget,
       );
     }
 

test/model/katex_test.dart

@@ -926,4 +926,4 @@ void main() async {
   }, skip: Platform.isWindows, // [intended] purely analyzes source, so
        // any one platform is enough; avoid dealing with Windows file paths
   );
-}
+}