Fix text field clipping when erasing rapidly. (#23894)

`RenderEditable.paint` assumes that if the length of the text fits within the
visible region, then the text will be rendered at the start of the region and be
completely visible. This is not always true, since the text may still be
rendered at an offset if an animation is ongoing when the text begins to fit.

This fixes #22288 and #14121

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

diff --git a/packages/flutter/lib/src/material/text_selection.dart b/packages/flutter/lib/src/material/text_selection.dart
index eba02b4..5f0ee5f 100644
--- a/packages/flutter/lib/src/material/text_selection.dart
+++ b/packages/flutter/lib/src/material/text_selection.dart
@@ -104,8 +104,7 @@
   }
 }
 
-/// Draws a single text selection handle. The [type] determines where the handle
-/// points (e.g. the [left] handle points up and to the right).
+/// Draws a single text selection handle which points up and to the left.
 class _TextSelectionHandlePainter extends CustomPainter {
   _TextSelectionHandlePainter({ this.color });
 

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

diff --git a/packages/flutter/lib/src/rendering/editable.dart b/packages/flutter/lib/src/rendering/editable.dart
index 551d379..663d3d5 100644
--- a/packages/flutter/lib/src/rendering/editable.dart
+++ b/packages/flutter/lib/src/rendering/editable.dart
@@ -921,7 +921,11 @@
     return null;
   }
 
-  bool _hasVisualOverflow = false;
+  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;
 
   /// Returns the local coordinates of the endpoints of the given selection.
   ///
@@ -1146,8 +1150,7 @@
     final Size textPainterSize = _textPainter.size;
     size = Size(constraints.maxWidth, constraints.constrainHeight(_preferredHeight(constraints.maxWidth)));
     final Size contentSize = Size(textPainterSize.width + _kCaretGap + cursorWidth, textPainterSize.height);
-    final double _maxScrollExtent = _getMaxScrollExtent(contentSize);
-    _hasVisualOverflow = _maxScrollExtent > 0.0;
+    _maxScrollExtent = _getMaxScrollExtent(contentSize);
     offset.applyViewportDimension(_viewportExtent);
     offset.applyContentDimensions(0.0, _maxScrollExtent);
   }

packages/flutter/test/rendering/editable_test.dart

diff --git a/packages/flutter/test/rendering/editable_test.dart b/packages/flutter/test/rendering/editable_test.dart
index 53b4ff0..c682d7b 100644
--- a/packages/flutter/test/rendering/editable_test.dart
+++ b/packages/flutter/test/rendering/editable_test.dart
@@ -6,6 +6,9 @@
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/services.dart';
 
+import '../rendering/mock_canvas.dart';
+import '../rendering/recording_canvas.dart';
+
 class FakeEditableTextState extends TextSelectionDelegate {
   @override
   TextEditingValue get textEditingValue { return const TextEditingValue(); }
@@ -65,4 +68,27 @@
       ),
     );
   });
+
+  // Test that clipping will be used even when the text fits within the visible
+  // region if the start position of the text is offset (e.g. during scrolling
+  // animation).
+  test('correct clipping', () {
+    final TextSelectionDelegate delegate = FakeEditableTextState();
+    final RenderEditable editable = RenderEditable(
+      text: const TextSpan(
+        style: TextStyle(height: 1.0, fontSize: 10.0, fontFamily: 'Ahem'),
+        text: 'A',
+      ),
+      textAlign: TextAlign.start,
+      textDirection: TextDirection.ltr,
+      locale: const Locale('en', 'US'),
+      offset: ViewportOffset.fixed(10.0),
+      textSelectionDelegate: delegate,
+    );
+    editable.layout(BoxConstraints.loose(const Size(1000.0, 1000.0)));
+    expect(
+      (Canvas canvas) => editable.paint(TestRecordingPaintingContext(canvas), Offset.zero),
+      paints..clipRect(rect: Rect.fromLTRB(0.0, 0.0, 1000.0, 10.0))
+    );
+  });
 }