Screen-tearing and other visual artifacts when scrolling the FigureCanvas

[ptziegler]

Consider the following snippet:

import org.eclipse.swt.SWT;
import org.eclipse.swt.graphics.Color;
import org.eclipse.swt.layout.FillLayout;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Shell;

import org.eclipse.draw2d.ColorConstants;
import org.eclipse.draw2d.Figure;
import org.eclipse.draw2d.FigureCanvas;
import org.eclipse.draw2d.FlowLayout;
import org.eclipse.draw2d.Graphics;
import org.eclipse.draw2d.IFigure;
import org.eclipse.draw2d.geometry.Dimension;

public class ScrollingBug {
	public static void main(String[] args) {
		System.setProperty("swt.autoScale.updateOnRuntime", "true");
		Shell shell = new Shell();
		shell.setSize(400, 400);
		shell.setLayout(new FillLayout());

		FigureCanvas canvas = new FigureCanvas(shell, SWT.NONE);
		Figure root = new Figure();
		root.setLayoutManager(new ListLayout());
		root.add(createLabel(ColorConstants.red));
		root.add(createLabel(ColorConstants.green));
		root.add(createLabel(ColorConstants.blue));
		root.add(createLabel(ColorConstants.yellow));
		root.add(createLabel(ColorConstants.cyan));
		root.add(createLabel(ColorConstants.white));
		root.add(createLabel(ColorConstants.gray));
		root.add(createLabel(ColorConstants.orange));
		canvas.getViewport().setContentsTracksHeight(true);
		canvas.getViewport().setContentsTracksWidth(true);
		canvas.getViewport().setContents(root);

		shell.open();

		Display display = shell.getDisplay();
		while (!display.isDisposed()) {
			display.readAndDispatch();
		}
	}

	private static IFigure createLabel(Color bg) {
		return new Figure() {
			@Override
			protected void paintFigure(Graphics graphics) {
				super.paintFigure(graphics);
				graphics.setBackgroundColor(bg);
				graphics.fillRectangle(getClientArea());
			}

			@Override
			public Dimension getPreferredSize(int wHint, int hHint) {
				return new Dimension(wHint, 100);
			}
		};
	}

	private static class ListLayout extends FlowLayout {
		public ListLayout() {
			setMajorSpacing(0);
		}
	}
}

To reproduce:

1. Launch the snippet on a system with > 100% zoom (in my case 150%)
2. Scroll the shell down to the bottom
3. Scroll the shell up to the top

What I expect:

The first line should be 150px tall.

What I get:

The first line is 154px tall.

The left shell is the original one with the 150px tall row, the right shell the one after scrolling with 154px.

Workaround:

Clicking on the canvas corrects the sizes for all figures. For non-trivial figures, this causes glitches and other visual artifacts. As a more "permanent" solution, one can hook a PropertyChangeListener to the scrollbar of the Viewport, that requests a repaint of the root figure, whenever the value changes.

[ptziegler]

Fixed via #770