flutter/packages/unit/test/rendering/overflow_test.dart

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'package:test/test.dart';

import 'rendering_tester.dart';

void main() {
  test("overflow should not affect baseline", () {
    RenderBox root, child, text;
    double baseline1, baseline2, height1, height2;

    root = new RenderPositionedBox(
      child: new RenderCustomPaint(
        child: child = text = new RenderParagraph(new PlainTextSpan('Hello World')),
        onPaint: (PaintingCanvas canvas, Size size) {
          baseline1 = child.getDistanceToBaseline(TextBaseline.alphabetic);
          height1 = text.size.height;
        }
      )
    );
    layout(root, phase: EnginePhase.paint);

    root = new RenderPositionedBox(
      child: new RenderCustomPaint(
        child: child = new RenderOverflowBox(
          child: text = new RenderParagraph(new PlainTextSpan('Hello World')),
          maxHeight: height1 / 2.0
        ),
        onPaint: (PaintingCanvas canvas, Size size) {
          baseline2 = child.getDistanceToBaseline(TextBaseline.alphabetic);
          height2 = text.size.height;
        }
      )
    );
    layout(root, phase: EnginePhase.paint);

    expect(baseline1, lessThan(height1));
    expect(height2, equals(height1 / 2.0));
    expect(baseline2, equals(baseline1));
    expect(baseline2, greaterThan(height2));
  });
}