Clamp scrollOffset to prevent textfield bouncing (#38573)

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

@@ -1017,6 +1017,14 @@ class RenderEditable extends RenderBox {
     return enableInteractiveSelection ?? !obscureText;
   }
 
+  /// The maximum amount the text is allowed to scroll.
+  ///
+  /// This value is only valid after layout and can change as additional
+  /// text is entered or removed in order to accommodate expanding when
+  /// [expands] is set to true.
+  double get maxScrollExtent => _maxScrollExtent;
+  double _maxScrollExtent = 0;
+
   double get _caretMargin => _kCaretGap + cursorWidth;
 
   @override
@@ -1229,8 +1237,6 @@ class RenderEditable extends RenderBox {
     return null;
   }
 
-  double _maxScrollExtent = 0;
-
   // We need to check the paint offset here because during animation, the start of
   // the text may position outside the visible region even when the text fits.
   bool get _hasVisualOverflow => _maxScrollExtent > 0 || _paintOffset != Offset.zero;

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

@@ -1286,6 +1286,7 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
       caretStart = caretRect.top - caretOffset;
       caretEnd = caretRect.bottom + caretOffset;
     } else {
+      // Scrolls horizontally for single-line fields.
       caretStart = caretRect.left;
       caretEnd = caretRect.right;
     }
@@ -1297,6 +1298,13 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
     } else if (caretEnd >= viewportExtent) { // cursor after end of bounds
       scrollOffset += caretEnd - viewportExtent;
     }
+
+    if (_isMultiline) {
+      // Clamp the final results to prevent programmatically scrolling to
+      // out-of-paragraph-bounds positions when encountering tall fonts/scripts that
+      // extend past the ascent.
+      scrollOffset = scrollOffset.clamp(0.0, renderEditable.maxScrollExtent);
+    }
     return scrollOffset;
   }
 

packages/flutter/test/rendering/editable_test.dart

@@ -544,4 +544,47 @@ void main() {
     editable.hasFocus = false;
     expect(editable.hasFocus, false);
   });
+
+  test('has correct maxScrollExtent', () {
+    final TextSelectionDelegate delegate = FakeEditableTextState();
+    EditableText.debugDeterministicCursor = true;
+
+    final RenderEditable editable = RenderEditable(
+      maxLines: 2,
+      backgroundCursorColor: Colors.grey,
+      textDirection: TextDirection.ltr,
+      cursorColor: const Color.fromARGB(0xFF, 0xFF, 0x00, 0x00),
+      offset: ViewportOffset.zero(),
+      textSelectionDelegate: delegate,
+      text: const TextSpan(
+        text: '撒地方加咖啡哈金凤凰卡号方式剪坏算法发挥福建垃\nasfjafjajfjaslfjaskjflasjfksajf撒分开建安路口附近拉设\n计费可使肌肤撒附近埃里克圾房卡设计费"',
+        style: TextStyle(
+          height: 1.0, fontSize: 10.0, fontFamily: 'Roboto',
+        ),
+      ),
+      startHandleLayerLink: LayerLink(),
+      endHandleLayerLink: LayerLink(),
+      selection: const TextSelection.collapsed(
+        offset: 4,
+        affinity: TextAffinity.upstream,
+      ),
+    );
+
+    editable.layout(BoxConstraints.loose(const Size(100.0, 1000.0)));
+    expect(editable.size, equals(const Size(100, 20)));
+    expect(editable.maxLines, equals(2));
+    expect(editable.maxScrollExtent, equals(90));
+
+    editable.layout(BoxConstraints.loose(const Size(150.0, 1000.0)));
+    expect(editable.maxScrollExtent, equals(50));
+
+    editable.layout(BoxConstraints.loose(const Size(200.0, 1000.0)));
+    expect(editable.maxScrollExtent, equals(40));
+
+    editable.layout(BoxConstraints.loose(const Size(500.0, 1000.0)));
+    expect(editable.maxScrollExtent, equals(10));
+
+    editable.layout(BoxConstraints.loose(const Size(1000.0, 1000.0)));
+    expect(editable.maxScrollExtent, equals(10));
+  });
 }

packages/flutter/test/widgets/editable_text_test.dart

@@ -2568,6 +2568,56 @@ void main() {
 
     debugDefaultTargetPlatformOverride = null;
   }, skip: isBrowser);
+
+  testWidgets('scrolling doesn\'t bounce', (WidgetTester tester) async {
+    // 3 lines of text, where the last line overflows and requires scrolling.
+    const String testText = 'XXXXX\nXXXXX\nXXXXX';
+    final TextEditingController controller = TextEditingController(text: testText);
+
+    await tester.pumpWidget(MaterialApp(
+      home: Align(
+        alignment: Alignment.topLeft,
+        child: SizedBox(
+          width: 100,
+          child: EditableText(
+            showSelectionHandles: true,
+            maxLines: 2,
+            controller: controller,
+            focusNode: FocusNode(),
+            style: Typography(platform: TargetPlatform.android).black.subhead.copyWith(fontFamily: 'Roboto'),
+            cursorColor: Colors.blue,
+            backgroundCursorColor: Colors.grey,
+            selectionControls: materialTextSelectionControls,
+            keyboardType: TextInputType.text,
+          ),
+        ),
+      ),
+    ));
+
+    final EditableTextState state =
+      tester.state<EditableTextState>(find.byType(EditableText));
+    final RenderEditable renderEditable = state.renderEditable;
+    final Scrollable scrollable = tester.widget<Scrollable>(find.byType(Scrollable));
+
+    expect(scrollable.controller.position.viewportDimension, equals(28));
+    expect(scrollable.controller.position.pixels, equals(0));
+
+    expect(renderEditable.maxScrollExtent, equals(14));
+
+    scrollable.controller.jumpTo(20.0);
+    await tester.pump();
+    expect(scrollable.controller.position.pixels, equals(20));
+
+    state.bringIntoView(const TextPosition(offset: 0));
+    await tester.pump();
+    expect(scrollable.controller.position.pixels, equals(0));
+
+
+    state.bringIntoView(const TextPosition(offset: 13));
+    await tester.pump();
+    expect(scrollable.controller.position.pixels, equals(14));
+    expect(scrollable.controller.position.pixels, equals(renderEditable.maxScrollExtent));
+  }, skip: isBrowser);
 }
 
 class MockTextSelectionControls extends Mock implements TextSelectionControls {