[win32] fallback for missing DPI change event

This commit adds a fallback mechanism if the OS doesn't send a DPI change
as expected. When the process is started with System DPI awareness and
only the thread is PerMonitorV2 aware, there are some scenarios, when the
OS does not send a DPI change event when a child Shell is positioned and
opened on another monitor as its parent Shell. To work around that limitation
a check is added to Shell::WM_WINDOWPOSCHANGED to trigger a dpi change
event if an unexpected DPI value is detected.

Author: akoch-yatta

bundles/org.eclipse.swt/Eclipse SWT Tests/win32/org/eclipse/swt/graphics/GCWin32Tests.java

@@ -16,7 +16,6 @@
 import static org.junit.Assert.assertEquals;
 
 import java.util.concurrent.*;
-import java.util.concurrent.atomic.*;
 
 import org.eclipse.swt.*;
 import org.eclipse.swt.internal.*;
@@ -30,30 +29,26 @@ class GCWin32Tests {
 
 	@Test
 	public void gcZoomLevelMustChangeOnShellZoomChange() {
-		Shell shell = new Shell(Display.getDefault());
+		checkGcZoomLevelOnCanvas(DPIUtil.getNativeDeviceZoom());
+		checkGcZoomLevelOnCanvas(DPIUtil.getNativeDeviceZoom()*2);
+	}
 
+	private void checkGcZoomLevelOnCanvas(int expectedZoom) {
+		Display display = Display.getDefault();
+		Shell shell = new Shell(display);
 		CompletableFuture<Integer> gcNativeZoom = new CompletableFuture<>();
-		CompletableFuture<Integer> scaledGcNativeZoom = new CompletableFuture<>();
-		int zoom = DPIUtil.getDeviceZoom();
-		AtomicBoolean isScaled = new AtomicBoolean(false);
-		shell.addListener(SWT.Paint, event -> {
-			if (isScaled.get()) {
-				scaledGcNativeZoom.complete(event.gc.getGCData().nativeZoom);
-			} else {
-				gcNativeZoom.complete(event.gc.getGCData().nativeZoom);
-			}
-		});
-
-		shell.open();
-		assertEquals("GCData must have a zoom level equal to the actual zoom level of the widget/shell", DPIUtil.getNativeDeviceZoom(), (int) gcNativeZoom.join());
 
-		int newSWTZoom = zoom * 2;
-		DPITestUtil.changeDPIZoom(shell, newSWTZoom);
-		isScaled.set(true);
-		shell.setVisible(false);
-		shell.setVisible(true);
+		Canvas canvas = new Canvas(shell, SWT.NONE);
+		canvas.setSize(20, 20);
+		shell.open ();
+		canvas.addPaintListener(event -> {
+			gcNativeZoom.complete(event.gc.getGCData().nativeZoom);
+		});
 
-		assertEquals("GCData must have a zoom level equal to the actual zoom level of the widget/shell on zoomChanged event", newSWTZoom, (int) scaledGcNativeZoom.join());
+		DPITestUtil.changeDPIZoom(shell, expectedZoom);
+		canvas.update();
+		assertEquals("GCData must have a zoom level equal to the actual zoom level of the widget/shell", expectedZoom, (int) gcNativeZoom.join());
+		shell.dispose();
 	}
 
 	@Test

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/Control.java

@@ -4928,21 +4928,24 @@ LRESULT WM_DESTROY (long wParam, long lParam) {
 	return null;
 }
 
+void handleMonitorSpecificDpiChange(int newNativeZoom, Rectangle newBoundsInPixels) {
+	DPIUtil.setDeviceZoom (newNativeZoom);
+
+	float scalingFactor = 1f * DPIUtil.getZoomForAutoscaleProperty(newNativeZoom) / DPIUtil.getZoomForAutoscaleProperty(nativeZoom);
+	DPIZoomChangeRegistry.applyChange(this, newNativeZoom, scalingFactor);
+
+	this.setBoundsInPixels(newBoundsInPixels.y, newBoundsInPixels.y, newBoundsInPixels.width, newBoundsInPixels.height);
+}
+
 LRESULT WM_DPICHANGED (long wParam, long lParam) {
 	// Map DPI to Zoom and compare
 	int newNativeZoom = DPIUtil.mapDPIToZoom (OS.HIWORD (wParam));
 	if (getDisplay().isRescalingAtRuntime()) {
 		Device.win32_destroyUnusedHandles(getDisplay());
-		int oldNativeZoom = nativeZoom;
-		if (newNativeZoom != oldNativeZoom) {
-			DPIUtil.setDeviceZoom (newNativeZoom);
-
-			float scalingFactor = 1f * DPIUtil.getZoomForAutoscaleProperty(newNativeZoom) / DPIUtil.getZoomForAutoscaleProperty(oldNativeZoom);
-			DPIZoomChangeRegistry.applyChange(this, newNativeZoom, scalingFactor);
-
+		if (newNativeZoom != nativeZoom) {
 			RECT rect = new RECT ();
 			COM.MoveMemory(rect, lParam, RECT.sizeof);
-			this.setBoundsInPixels(rect.left, rect.top, rect.right - rect.left, rect.bottom-rect.top);
+			handleMonitorSpecificDpiChange(newNativeZoom, new Rectangle(rect.left, rect.top, rect.right - rect.left, rect.bottom-rect.top));
 			return LRESULT.ZERO;
 		}
 	} else {

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/Shell.java

@@ -2685,6 +2685,24 @@ LRESULT WM_WINDOWPOSCHANGING (long wParam, long lParam) {
 	return result;
 }
 
+@Override
+LRESULT WM_WINDOWPOSCHANGED (long wParam, long lParam) {
+	LRESULT result = super.WM_WINDOWPOSCHANGED(wParam, lParam);
+	// When the process is started with System DPI awareness and
+	// only the thread is PerMonitorV2 aware, there are some scenarios, when the
+	// OS does not send a DPI change event when a child Shell is positioned and
+	// opened on another monitor as its parent Shell. To work around that limitation
+	// this check is added to trigger a dpi change event if an unexpected DPI value is
+	// detected.
+	if (display.isRescalingAtRuntime()) {
+		int dpiForWindow = DPIUtil.mapDPIToZoom(OS.GetDpiForWindow(getShell().handle));
+		if (dpiForWindow != nativeZoom) {
+			handleMonitorSpecificDpiChange(dpiForWindow, DPIUtil.scaleUp(getBounds(), dpiForWindow));
+		}
+	}
+	return result;
+}
+
 private static void handleDPIChange(Widget widget, int newZoom, float scalingFactor) {
 	if (!(widget instanceof Shell shell)) {
 		return;