Fix cursor outside of input width (#30525)

* Disallow cursor from appearing beyond the width of the input.

* Test that verifies the cursor can't exceed the width of the input

* Use constant from editable.dart to explain 1 pixel difference in test

* Fix failing test that tested the case of overflowing spaces

packages/flutter/lib/src/painting/text_painter.dart

@@ -486,7 +486,7 @@ class TextPainter {
 
       final double caretEnd = box.end;
       final double dx = box.direction == TextDirection.rtl ? caretEnd - caretPrototype.width : caretEnd;
-      return Offset(dx, box.top);
+      return Offset(min(dx, width), box.top);
     }
     return null;
   }
@@ -526,7 +526,7 @@ class TextPainter {
       final TextBox box = boxes.last;
       final double caretStart = box.start;
       final double dx = box.direction == TextDirection.rtl ? caretStart - caretPrototype.width : caretStart;
-      return Offset(dx, box.top);
+      return Offset(min(dx, width), box.top);
     }
     return null;
   }

packages/flutter/test/material/text_field_test.dart

@@ -426,6 +426,82 @@ void main() {
   }, skip: !Platform.isLinux);
   */
 
+  testWidgets('Overflowing a line with spaces stops the cursor at the end', (WidgetTester tester) async {
+    final TextEditingController controller = TextEditingController();
+
+    await tester.pumpWidget(
+      overlay(
+        child: TextField(
+          key: textFieldKey,
+          controller: controller,
+          maxLines: null,
+        ),
+      )
+    );
+    expect(controller.selection.baseOffset, -1);
+    expect(controller.selection.extentOffset, -1);
+
+    const String testValueOneLine = 'enough text to be exactly at the end of the line.';
+    await tester.enterText(find.byType(TextField), testValueOneLine);
+    await skipPastScrollingAnimation(tester);
+
+    RenderBox findInputBox() => tester.renderObject(find.byKey(textFieldKey));
+
+    RenderBox inputBox = findInputBox();
+    final Size oneLineInputSize = inputBox.size;
+
+    await tester.tapAt(textOffsetToPosition(tester, testValueOneLine.length));
+    await tester.pump();
+
+    const String testValueTwoLines = 'enough text to overflow the first line and go to the second';
+    await tester.enterText(find.byType(TextField), testValueTwoLines);
+    await skipPastScrollingAnimation(tester);
+
+    expect(inputBox, findInputBox());
+    inputBox = findInputBox();
+    expect(inputBox.size.height, greaterThan(oneLineInputSize.height));
+    final Size twoLineInputSize = inputBox.size;
+
+    // Enter a string with the same number of characters as testValueTwoLines,
+    // but where the overflowing part is all spaces. Assert that it only renders
+    // on one line.
+    const String testValueSpaces = testValueOneLine + '          ';
+    expect(testValueSpaces.length, testValueTwoLines.length);
+    await tester.enterText(find.byType(TextField), testValueSpaces);
+    await skipPastScrollingAnimation(tester);
+
+    expect(inputBox, findInputBox());
+    inputBox = findInputBox();
+    expect(inputBox.size.height, oneLineInputSize.height);
+
+    // Swapping the final space for a letter causes it to wrap to 2 lines.
+    const String testValueSpacesOverflow = testValueOneLine + '         a';
+    expect(testValueSpacesOverflow.length, testValueTwoLines.length);
+    await tester.enterText(find.byType(TextField), testValueSpacesOverflow);
+    await skipPastScrollingAnimation(tester);
+
+    expect(inputBox, findInputBox());
+    inputBox = findInputBox();
+    expect(inputBox.size.height, twoLineInputSize.height);
+
+    // Positioning the cursor at the end of a line overflowing with spaces puts
+    // it inside the input still.
+    await tester.enterText(find.byType(TextField), testValueSpaces);
+    await skipPastScrollingAnimation(tester);
+    await tester.tapAt(textOffsetToPosition(tester, testValueSpaces.length));
+    await tester.pump();
+
+    final double inputWidth = findRenderEditable(tester).size.width;
+    final Offset cursorOffsetSpaces = findRenderEditable(tester).getLocalRectForCaret(
+      const TextPosition(offset: testValueSpaces.length),
+    ).bottomRight;
+
+    // Gap between caret and edge of input, defined in editable.dart.
+    const int _kCaretGap = 1;
+
+    expect(cursorOffsetSpaces.dx, inputWidth - _kCaretGap);
+  });
+
   testWidgets('obscureText control test', (WidgetTester tester) async {
     await tester.pumpWidget(
       overlay(

packages/flutter/test/painting/text_painter_test.dart

@@ -133,7 +133,9 @@ void main() {
     Offset caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 0), ui.Rect.zero);
     expect(caretOffset.dx, 21);
     caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: text.length), ui.Rect.zero);
-    expect(caretOffset.dx, 441);
+    // The end of the line is 441, but the width is only 420, so the cursor is
+    // stopped there without overflowing.
+    expect(caretOffset.dx, painter.width);
 
     caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 1), ui.Rect.zero);
     expect(caretOffset.dx, 35);