Stop TextFields from wobbling within scrollables (#24015)

packages/flutter/lib/src/rendering/viewport.dart

@@ -984,11 +984,7 @@ abstract class RenderViewportBase<ParentDataClass extends ContainerParentDataMix
 
     assert(targetOffset != null);
 
-    if (duration == Duration.zero) {
-      offset.jumpTo(targetOffset.offset);
-    } else {
-      offset.animateTo(targetOffset.offset, duration: duration, curve: curve);
-    }
+    offset.moveTo(targetOffset.offset, duration: duration, curve: curve);
     return targetOffset.rect;
   }
 }

packages/flutter/lib/src/rendering/viewport_offset.dart

@@ -181,6 +181,27 @@ abstract class ViewportOffset extends ChangeNotifier {
     @required Curve curve,
   });
 
+  /// Calls [jumpTo] if duration is null or [Duration.zero], otherwise
+  /// [animateTo] is called.
+  ///
+  /// If [animateTo] is called then [curve] defaults to [Curves.ease]. The
+  /// [clamp] parameter is ignored by this stub implementation but subclasses
+  /// like [ScrollPosition] handle it by adjusting [to] to prevent over or
+  /// underscroll.
+  Future<void> moveTo(double to, {
+    Duration duration,
+    Curve curve,
+    bool clamp,
+  }) {
+    assert(to != null);
+    if (duration == null || duration == Duration.zero) {
+      jumpTo(to);
+      return Future<void>.value();
+    } else {
+      return animateTo(to, duration: duration, curve: curve ?? Curves.ease);
+    }
+  }
+
   /// The direction in which the user is trying to change [pixels], relative to
   /// the viewport's [RenderViewport.axisDirection].
   ///

packages/flutter/lib/src/widgets/scroll_position.dart

@@ -562,6 +562,28 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
   @override
   void jumpTo(double value);
 
+  /// Calls [jumpTo] if duration is null or [Duration.zero], otherwise
+  /// [animateTo] is called.
+  ///
+  /// If [clamp] is true (the default) then [to] is adjusted to prevent over or
+  /// underscroll.
+  ///
+  /// If [animateTo] is called then [curve] defaults to [Curves.ease].
+  @override
+  Future<void> moveTo(double to, {
+    Duration duration,
+    Curve curve,
+    bool clamp = true,
+  }) {
+    assert(to != null);
+    assert(clamp != null);
+
+    if (clamp)
+      to = to.clamp(minScrollExtent, maxScrollExtent);
+
+    return super.moveTo(to, duration: duration, curve: curve);
+  }
+
   @override
   bool get allowImplicitScrolling => physics.allowImplicitScrolling;
 

packages/flutter/test/material/text_field_test.dart

@@ -4,6 +4,7 @@
 
 import 'dart:async';
 import 'dart:io' show Platform;
+import 'dart:math' as math;
 import 'dart:ui' as ui show window;
 
 import 'package:flutter/material.dart';
@@ -3182,4 +3183,148 @@ void main() {
     final Rect fieldRect = tester.getRect(find.text('Just some text'));
     expect(labelRect.bottom, lessThanOrEqualTo(fieldRect.top));
   });
+
+  testWidgets('TextField scrolls into view but does not bounce (SingleChildScrollView)', (WidgetTester tester) async {
+    // This is a regression test for https://github.com/flutter/flutter/issues/20485
+
+    final Key textField1 = UniqueKey();
+    final Key textField2 = UniqueKey();
+    final ScrollController scrollController = ScrollController();
+
+    double minOffset;
+    double maxOffset;
+
+    scrollController.addListener(() {
+      final double offset = scrollController.offset;
+      minOffset = math.min(minOffset ?? offset, offset);
+      maxOffset = math.max(maxOffset ?? offset, offset);
+    });
+
+    Widget buildFrame(Axis scrollDirection) {
+      return MaterialApp(
+        home: Scaffold(
+          body: SafeArea(
+            child: SingleChildScrollView(
+              physics: const BouncingScrollPhysics(),
+              controller: scrollController,
+              child: Column(
+                children: <Widget>[
+                  SizedBox( // visible when scrollOffset is 0.0
+                    height: 100.0,
+                    width: 100.0,
+                    child: TextField(key: textField1, scrollPadding: const EdgeInsets.all(200.0)),
+                  ),
+                  const SizedBox(
+                    height: 600.0, // Same size as the frame. Initially
+                    width: 800.0,  // textField2 is not visible
+                  ),
+                  SizedBox( // visible when scrollOffset is 200.0
+                    height: 100.0,
+                    width: 100.0,
+                    child: TextField(key: textField2, scrollPadding: const EdgeInsets.all(200.0)),
+                  ),
+                ],
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildFrame(Axis.vertical));
+    await tester.enterText(find.byKey(textField1), '1');
+    await tester.pumpAndSettle();
+    await tester.enterText(find.byKey(textField2), '2'); //scroll textField2 into view
+    await tester.pumpAndSettle();
+    await tester.enterText(find.byKey(textField1), '3'); //scroll textField1 back into view
+    await tester.pumpAndSettle();
+
+    expect(minOffset, 0.0);
+    expect(maxOffset, 200.0);
+
+    minOffset = null;
+    maxOffset = null;
+
+    await tester.pumpWidget(buildFrame(Axis.horizontal));
+    await tester.enterText(find.byKey(textField1), '1');
+    await tester.pumpAndSettle();
+    await tester.enterText(find.byKey(textField2), '2'); //scroll textField2 into view
+    await tester.pumpAndSettle();
+    await tester.enterText(find.byKey(textField1), '3'); //scroll textField1 back into view
+    await tester.pumpAndSettle();
+
+    expect(minOffset, 0.0);
+    expect(maxOffset, 200.0);
+  });
+
+  testWidgets('TextField scrolls into view but does not bounce (ListView)', (WidgetTester tester) async {
+    // This is a regression test for https://github.com/flutter/flutter/issues/20485
+
+    final Key textField1 = UniqueKey();
+    final Key textField2 = UniqueKey();
+    final ScrollController scrollController = ScrollController();
+
+    double minOffset;
+    double maxOffset;
+
+    scrollController.addListener(() {
+      final double offset = scrollController.offset;
+      minOffset = math.min(minOffset ?? offset, offset);
+      maxOffset = math.max(maxOffset ?? offset, offset);
+    });
+
+    Widget buildFrame(Axis scrollDirection) {
+      return MaterialApp(
+        home: Scaffold(
+          body: SafeArea(
+            child: ListView(
+              physics: const BouncingScrollPhysics(),
+              controller: scrollController,
+              children: <Widget>[
+                SizedBox( // visible when scrollOffset is 0.0
+                  height: 100.0,
+                  width: 100.0,
+                  child: TextField(key: textField1, scrollPadding: const EdgeInsets.all(200.0)),
+                ),
+                const SizedBox(
+                  height: 450.0, // 50.0 smaller than the overall frame so that both
+                  width: 650.0,  // textfields are always partially visible.
+                ),
+                SizedBox( // visible when scrollOffset = 50.0
+                  height: 100.0,
+                  width: 100.0,
+                  child: TextField(key: textField2, scrollPadding: const EdgeInsets.all(200.0)),
+                ),
+              ],
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildFrame(Axis.vertical));
+    await tester.enterText(find.byKey(textField1), '1'); // textfield1 is visible
+    await tester.pumpAndSettle();
+    await tester.enterText(find.byKey(textField2), '2'); //scroll textField2 into view
+    await tester.pumpAndSettle();
+    await tester.enterText(find.byKey(textField1), '3'); //scroll textField1 back into view
+    await tester.pumpAndSettle();
+
+    expect(minOffset, 0.0);
+    expect(maxOffset, 50.0);
+
+    minOffset = null;
+    maxOffset = null;
+
+    await tester.pumpWidget(buildFrame(Axis.horizontal));
+    await tester.enterText(find.byKey(textField1), '1'); // textfield1 is visible
+    await tester.pumpAndSettle();
+    await tester.enterText(find.byKey(textField2), '2'); //scroll textField2 into view
+    await tester.pumpAndSettle();
+    await tester.enterText(find.byKey(textField1), '3'); //scroll textField1 back into view
+    await tester.pumpAndSettle();
+
+    expect(minOffset, 0.0);
+    expect(maxOffset, 50.0);
+  });
 }