Fix OutlineInputBorder crash (#30123)

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

@@ -412,7 +412,7 @@ class OutlineInputBorder extends InputBorder {
 
     const double cornerArcSweep = math.pi / 2.0;
     final double tlCornerArcSweep = start < center.tlRadiusX
-      ? math.asin(start / center.tlRadiusX)
+      ? math.asin((start / center.tlRadiusX).clamp(-1.0, 1.0))
       : math.pi / 2.0;
 
     final Path path = Path()
@@ -474,12 +474,12 @@ class OutlineInputBorder extends InputBorder {
       final double extent = lerpDouble(0.0, gapExtent + gapPadding * 2.0, gapPercentage);
       switch (textDirection) {
         case TextDirection.rtl: {
-          final Path path = _gapBorderPath(canvas, center, gapStart + gapPadding - extent, extent);
+          final Path path = _gapBorderPath(canvas, center, math.max(0.0, gapStart + gapPadding - extent), extent);
           canvas.drawPath(path, paint);
           break;
         }
         case TextDirection.ltr: {
-          final Path path = _gapBorderPath(canvas, center, gapStart - gapPadding, extent);
+          final Path path = _gapBorderPath(canvas, center, math.max(0.0, gapStart - gapPadding), extent);
           canvas.drawPath(path, paint);
           break;
         }

packages/flutter/test/material/input_decorator_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:async';
 import 'dart:io' show Platform;
 
 import 'package:flutter/material.dart';
@@ -2255,6 +2256,41 @@ void main() {
     expect(getBorderRadius(tester), BorderRadius.zero);
   });
 
+  testWidgets('OutlineInputBorder async lerp', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/28724
+
+    final Completer<void> completer = Completer<void>();
+    bool waitIsOver = false;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return GestureDetector(
+              onTap: () async {
+                setState(() { waitIsOver = true; });
+                await completer.future;
+                setState(() { waitIsOver = false;  });
+              },
+              child: InputDecorator(
+                decoration: InputDecoration(
+                  labelText: 'Test',
+                  enabledBorder: !waitIsOver ? null : const OutlineInputBorder(borderSide: BorderSide(color: Colors.blue)),
+                ),
+              ),
+            );
+          },
+        ),
+      ),
+    );
+
+    await tester.tap(find.byType(StatefulBuilder));
+    await tester.pumpAndSettle();
+
+    completer.complete();
+    await tester.pumpAndSettle();
+  });
+
   test('InputBorder equality', () {
     // OutlineInputBorder's equality is defined by the borderRadius, borderSide, & gapPadding
     const OutlineInputBorder outlineInputBorder = OutlineInputBorder(