小部件固定在滚动小部件上方

Question

我正在尝试将搜索栏添加到StyledText小部件的自定义版本中。我希望酒吧卡在右上角。我创建了一个你可以尝试的最小测试。

package test;

import org.eclipse.swt.SWT;
import org.eclipse.swt.custom.StyledText;
import org.eclipse.swt.events.PaintEvent;
import org.eclipse.swt.events.PaintListener;
import org.eclipse.swt.graphics.Color;
import org.eclipse.swt.graphics.Point;
import org.eclipse.swt.layout.GridData;
import org.eclipse.swt.layout.GridLayout;
import org.eclipse.swt.widgets.Canvas;
import org.eclipse.swt.widgets.Composite;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Shell;

public class WidgetTest {
    protected final Display display = Display.getDefault();
    protected Shell shell;

    public static void main(String[] args) {
        WidgetTest window = new WidgetTest();
        window.open();
    }

    private void open() {
        createContents();
        shell.open();
        shell.layout();
        while (!shell.isDisposed()) {
            if (!display.readAndDispatch()) {
                display.sleep();
            }
        }
    }

    protected void createContents() {
        shell = new Shell();
        shell.setSize(300, 178);
        shell.setLayout(new GridLayout(1, false));
        new CustomText(shell, SWT.V_SCROLL | SWT.MULTI | SWT.H_SCROLL)
           .setLayoutData(new GridData(SWT.FILL, SWT.FILL, true, true, 1, 1));
    }

    private class CustomText extends StyledText {

        private class CustomWidget extends Canvas {
            public CustomWidget(Composite parent, int style) {
                super(parent, style);
                addPaintListener(new PaintListener() {

                    @Override
                    public void paintControl(PaintEvent e) {
                        e.gc.fillRectangle(e.gc.getClipping());
                    }
                });
            }
        }

        private CustomWidget widget;

        public CustomText(Composite parent, int style) {
            super(parent, style);
            widget = new CustomWidget(this, SWT.NONE);
            widget.setBackground(new Color(getDisplay(), 255,0,0));
        }

        @Override
        public void setBounds(int x, int y, int width, int height) {
            super.setBounds(x, y, width, height);
            Point p = getSize();
            widget.setBounds(p.x - 200 - 20, 0, 200, 25);
            widget.moveAbove(CustomText.this);
        }

    }

}

红色框代表搜索框。创建窗口或重新调整大小时，框会转到正确的位置。但是，当您键入文本或滚动时，框会移动。

我尝试添加一个绘图侦听器并重置该框的位置。虽然它主要起作用，但它在图形上是微不足道的，并且在正确定位之前搜索框会移动一两帧。

addPaintListener(new PaintListener() {
    @Override
    public void paintControl(PaintEvent arg0) {
        Point p = getSize();
        widget.setBounds(p.x - 200 - 20, 0, 200, 25);
    }
});

如何正确地将小部件固定到顶角？