Handle HiDPI scaling in Zest Graph

This replicates the HiDPI scaling in GEF diagrams that was introduced
with 1bd2883, albeit in a simplified
form. Zest doesn't have an EditPart viewer and instead, the root layer
is directly embedded into the Graph widget.

When the native auto-scaling provided by SWT is disabled for the Graph,
a zoom listener is added that automatically synchronizes the monitor
zoom of the widget with the scale of the root figure.

Author: ptziegler

org.eclipse.draw2d/src/org/eclipse/draw2d/internal/InternalDraw2dUtils.java

@@ -29,6 +29,11 @@ public class InternalDraw2dUtils {
 	 */
 	private static final String DRAW2D_DISABLE_AUTOSCALE = "draw2d.disableAutoscale"; //$NON-NLS-1$
 
+	/**
+	 * Internal flag for fetching the shell zoom
+	 */
+	private static final String DATA_SHELL_ZOOM = "SHELL_ZOOM"; //$NON-NLS-1$
+
 	public static boolean disableAutoscale;
 
 	static {
@@ -41,4 +46,17 @@ public static void configureForAutoscalingMode(Control control) {
 			control.setData("AUTOSCALE_DISABLED", true); //$NON-NLS-1$
 		}
 	}
+
+	public static double calculateScale(Control control) {
+		if (!InternalDraw2dUtils.disableAutoscale || control == null) {
+			return 1.0;
+		}
+		int shellZooom;
+		try {
+			shellZooom = (int) control.getData(InternalDraw2dUtils.DATA_SHELL_ZOOM);
+		} catch (ClassCastException | NullPointerException e) {
+			shellZooom = 100;
+		}
+		return shellZooom / 100.0;
+	}
 }

org.eclipse.gef/src/org/eclipse/gef/ui/parts/AbstractEditPartViewer.java

@@ -118,11 +118,6 @@ public abstract class AbstractEditPartViewer implements EditPartViewer {
 	private KeyHandler keyHandler;
 	private PropertyChangeSupport changeSupport;
 
-	/**
-	 * Internal flag for fetching the shell zoom
-	 */
-	private static final String DATA_SHELL_ZOOM = "SHELL_ZOOM"; //$NON-NLS-1$
-
 	/**
 	 * Constructs the viewer and calls {@link #init()}.
 	 */
@@ -496,7 +491,7 @@ protected void hookControl() {
 			control.addListener(SWT.ZoomChanged,
 					e -> setProperty(InternalGEFPlugin.MONITOR_SCALE_PROPERTY, e.detail / 100.0));
 		}
-		setProperty(InternalGEFPlugin.MONITOR_SCALE_PROPERTY, calculateScale());
+		setProperty(InternalGEFPlugin.MONITOR_SCALE_PROPERTY, InternalDraw2dUtils.calculateScale(control));
 	}
 
 	/**
@@ -867,17 +862,4 @@ protected void unhookControl() {
 	@Override
 	public void unregisterAccessibleEditPart(AccessibleEditPart acc) {
 	}
-
-	private double calculateScale() {
-		if (!InternalDraw2dUtils.disableAutoscale || control == null) {
-			return 1.0;
-		}
-		int shellZooom;
-		try {
-			shellZooom = (int) control.getData(DATA_SHELL_ZOOM);
-		} catch (ClassCastException | NullPointerException e) {
-			shellZooom = 100;
-		}
-		return shellZooom / 100.0;
-	}
 }

org.eclipse.zest.core/src/org/eclipse/zest/core/widgets/Graph.java

@@ -74,6 +74,7 @@
 import org.eclipse.draw2d.geometry.Point;
 import org.eclipse.draw2d.geometry.Rectangle;
 import org.eclipse.draw2d.internal.FileImageDataProvider;
+import org.eclipse.draw2d.internal.InternalDraw2dUtils;
 
 /**
  * Holds the nodes and connections for the graph.
@@ -239,6 +240,12 @@ public void dispatchMouseMoved(org.eclipse.swt.events.MouseEvent me) {
 		this.zoomListener = new ZoomGestureListener();
 		this.rotateListener = new RotateGestureListener();
 
+		if (InternalDraw2dUtils.disableAutoscale) {
+			InternalDraw2dUtils.configureForAutoscalingMode(this);
+			addListener(SWT.ZoomChanged, event -> rootlayer.setScale(event.detail / 100.0));
+			rootlayer.setScale(InternalDraw2dUtils.calculateScale(this));
+		}
+
 		this.addPaintListener(event -> {
 			if (shouldSheduleLayout) {
 				applyLayoutInternal(true);

org.eclipse.zest.doc.isv/guide-src/graph.adoc

@@ -96,4 +96,23 @@ Draw2D.
 
 NOTE: Graph connections are always added to the _Graph_, regardless of whether
 the node are inside a sub-graph. This means that it is possible to e.g. connect
-two vertices in separate sub-graphs.
+two vertices in separate sub-graphs.
+
+== HighDPI Support
+
+The `*Figures*` used for visualization of the `*GraphItems*` are painted
+using absolute coordinates. In combination with fractional scaling, this may
+result in rounding errors, causing visual glitches in the graph.
+
+By setting the `*draw2d.disableAutoscale*` system property to `*true*`, the
+native scaling capability of the underlying `*Graph*`, which is normally
+provided by SWT, is disabled and instead done by Draw2D.
+
+When a `*Graph*` is created, a `*ZoomChanged*` listener is registered. The
+current monitor zoom (and any changes to it) are used as scale for the
+`*RootLayer*`.
+
+image:images/highdpi.svg[Relation between the FigureCanvas, EditPartViewer and RootEditPart with respect to the device zoom]
+
+NOTE: Fractional scaling is currently only supported for Windows. On Linux and
+MacOS, this system property is ignored.