Text field border radius bug fix (#24414)

* Fix radius glitch by carrying over border when lerping outline borders

* Test that border doesn't change

* Regression comment

packages/flutter/lib/src/material/input_border.dart

@@ -195,6 +195,7 @@ class UnderlineInputBorder extends InputBorder {
     if (a is UnderlineInputBorder) {
       return UnderlineInputBorder(
         borderSide: BorderSide.lerp(a.borderSide, borderSide, t),
+        borderRadius: BorderRadius.lerp(a.borderRadius, borderRadius, t),
       );
     }
     return super.lerpFrom(a, t);
@@ -205,6 +206,7 @@ class UnderlineInputBorder extends InputBorder {
     if (b is UnderlineInputBorder) {
       return UnderlineInputBorder(
         borderSide: BorderSide.lerp(borderSide, b.borderSide, t),
+        borderRadius: BorderRadius.lerp(borderRadius, b.borderRadius, t),
       );
     }
     return super.lerpTo(b, t);

packages/flutter/lib/src/material/input_decorator.dart

@@ -1898,10 +1898,10 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
         helperError: helperError,
         counter: counter,
         container: container,
-    ),
-    textDirection: textDirection,
-    textBaseline: textBaseline,
-    isFocused: isFocused,
+      ),
+      textDirection: textDirection,
+      textBaseline: textBaseline,
+      isFocused: isFocused,
     );
   }
 }

packages/flutter/test/material/input_decorator_test.dart

@@ -77,6 +77,14 @@ BorderSide getBorderSide(WidgetTester tester) {
   return getBorder(tester)?.borderSide;
 }
 
+BorderRadius getBorderRadius(WidgetTester tester) {
+  final InputBorder border = getBorder(tester);
+  if (border is UnderlineInputBorder) {
+    return border.borderRadius;
+  }
+  return null;
+}
+
 double getBorderWeight(WidgetTester tester) => getBorderSide(tester)?.width;
 
 Color getBorderColor(WidgetTester tester) => getBorderSide(tester)?.color;
@@ -1931,6 +1939,44 @@ void main() {
     expect(getBorder(tester), disabledBorder);
   });
 
+  testWidgets('OutlineInputBorder radius carries over when lerping', (WidgetTester tester) async {
+    // This is a regression test for https://github.com/flutter/flutter/issues/23982
+    const Key key = Key('textField');
+
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Material(
+          child: Directionality(
+            textDirection: TextDirection.ltr,
+            child: TextField(
+              key: key,
+              decoration: InputDecoration(
+                fillColor: Colors.white,
+                filled: true,
+                border: UnderlineInputBorder(
+                  borderSide: BorderSide(color: Colors.blue, width: 2.0),
+                  borderRadius: BorderRadius.zero,
+                ),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    // TextField has the given border
+    expect(getBorderRadius(tester), BorderRadius.zero);
+
+    // Focusing does not change the border
+    await tester.tap(find.byKey(key));
+    await tester.pump();
+    expect(getBorderRadius(tester), BorderRadius.zero);
+    await tester.pump(const Duration(milliseconds: 100));
+    expect(getBorderRadius(tester), BorderRadius.zero);
+    await tester.pumpAndSettle();
+    expect(getBorderRadius(tester), BorderRadius.zero);
+  });
+
   test('InputBorder equality', () {
     // OutlineInputBorder's equality is defined by the borderRadius, borderSide, & gapPadding
     const OutlineInputBorder outlineInputBorder = OutlineInputBorder(
@@ -1971,5 +2017,4 @@ void main() {
     expect(underlineInputBorder.hashCode, const UnderlineInputBorder(borderSide: BorderSide(color: Colors.blue)).hashCode);
     expect(underlineInputBorder.hashCode, isNot(const UnderlineInputBorder().hashCode));
   });
-
 }