Showing lean windows makes them tiny

Run the supplied RobotTest regression case first, then read org.assertj.swing.monitor.WindowStatus#makeLargeEnoughToReceiveEvents and its shouldResize() logic. Done means showWindow leaves a packed window taller than its title-bar inset instead of shrinking it to MINIMUM_WINDOW_SIZE, and the regression test passes.

If a Window has little content, AssertJ makes the window even smaller.

Here's a failing test:

import org.assertj.swing.core.BasicRobot;
import org.assertj.swing.core.Robot;
import org.assertj.swing.edt.GuiActionRunner;
import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import org.mockito.ArgumentCaptor;

import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JTextField;
import java.awt.Dimension;
import java.util.List;

import static org.assertj.core.api.Assertions.within;
import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
import static org.assertj.core.api.Assumptions.assumeThat;
import static org.mockito.BDDMockito.then;
import static org.mockito.Mockito.atLeastOnce;
import static org.mockito.Mockito.spy;


class RobotTest {

    private Robot robot;

    @BeforeEach
    void setUp() {
        robot = BasicRobot.robotWithCurrentAwtHierarchy();
    }

    @AfterEach
    void tearDown() {
        robot.cleanUp();
    }

    @Test
    void showWindow_ifPreferredWindowHeightExceedsTitleBarHeight_doesNotShrinkWindowToTitleBarHeight() {
        JTextField field = new JTextField();
        field.setColumns(10);
        JFrame frame = spy(createFrame(field));
        int titleBarInset = frame.getInsets().top;
        assumeThat(titleBarInset).isCloseTo(30, within(1));
        assumeThat(frame.getPreferredSize().height).isGreaterThan(titleBarInset);

        robot.showWindow(frame);

        ArgumentCaptor<Dimension> captor = ArgumentCaptor.forClass(Dimension.class);
        then(frame).should(atLeastOnce()).setSize(captor.capture());
        List<Dimension> setSizes = captor.getAllValues();
        Dimension lastSetDimension = setSizes.get(setSizes.size() - 1);
        assertThat(lastSetDimension.height).isGreaterThan(titleBarInset);
    }

    private JFrame createFrame(JComponent component) {
        return GuiActionRunner.execute(() -> {
            JFrame frame = new JFrame();
            frame.add(component);
            frame.pack(); // packed
            return frame;
        });
    }
}

It's all because of the shouldResize() method (see org.assertj.swing.monitor.WindowStatus#makeLargeEnoughToReceiveEvents). If the window's preferred size (which doesn't include insets) minus ≈30 pixels of the title bar top inset is lower than 30 pixels, the window is guaranteed to be shrunk to MINIMUM_WINDOW_SIZE (50,30). You also get ugly jittering of the pointer for several seconds.

// org.assertj.swing.monitor.WindowStatus#makeLargeEnoughToReceiveEvents
  @RunsInCurrentThread
  private void makeLargeEnoughToReceiveEvents(@Nonnull Window window) {
    if (!shouldResize(window)) {
      return;
    }
    window.setSize(MINIMUM_WINDOW_SIZE);
  }

Java 8.