[Win32] Fix wrong layout when opening shell or reparenting control #2608

In multiple situation, relayouting due to DPI change events does not
work as expected:
- When opening child shells, they may be relayouted after being opened
if their position changed to a different monitor between the OS handle
was created and a new position was set
- When reparenting controls between different shells, the target shell
may not be properly relayout at all

Since the user experience when performing these two cases asynchronously
is not good anyway (e.g., reparenting leads to unexpected asynchronous
relayouting operations), this change adapts the two use cases to process
the DPI change synchronously.

Fixes #2608

Author: HeikoKlare

bundles/org.eclipse.swt/Eclipse SWT Tests/win32/org/eclipse/swt/widgets/DPITestUtil.java

@@ -28,7 +28,7 @@ private DPITestUtil() {
 
 	public static void changeDPIZoom (Shell shell, int nativeZoom) {
 		DPIUtil.setDeviceZoom(nativeZoom);
-		Event event = shell.createZoomChangedEvent(nativeZoom);
+		Event event = shell.createZoomChangedEvent(nativeZoom, true);
 		shell.sendZoomChangedEvent(event, shell);
 		DPIChangeExecution data = (DPIChangeExecution) event.data;
 		waitForDPIChange(shell, TIMEOUT_MILLIS, data.taskCount);

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

@@ -4801,12 +4801,9 @@ public boolean setParent (Composite parent) {
 	if (OS.SetParent (topHandle, parent.handle) == 0) return false;
 	this.parent = parent;
 	// If parent changed, zoom level might need to be adjusted
-	if (parent.nativeZoom != nativeZoom) {
-		int newZoom = parent.nativeZoom;
-		Event zoomChangedEvent = createZoomChangedEvent(newZoom);
-		if (currentDpiChangeEvent != null) {
-			currentDpiChangeEvent.doit = false;
-		}
+	int newZoom = parent.nativeZoom;
+	if (newZoom != nativeZoom) {
+		Event zoomChangedEvent = createZoomChangedEvent(newZoom, false);
 		sendZoomChangedEvent(zoomChangedEvent, getShell());
 	}
 	int flags = OS.SWP_NOSIZE | OS.SWP_NOMOVE | OS.SWP_NOACTIVATE;
@@ -5019,7 +5016,10 @@ LRESULT WM_DESTROY (long wParam, long lParam) {
 
 private void handleMonitorSpecificDpiChange(int newNativeZoom, Rectangle newBoundsInPixels) {
 	DPIUtil.setDeviceZoom (newNativeZoom);
-	Event zoomChangedEvent = createZoomChangedEvent(newNativeZoom);
+	// Do not process DPI change for child shells asynchronous to avoid relayouting when
+	// repositioning the child shell to a different monitor upon opening
+	boolean processDpiChangeAsynchronous = getShell().getParent() == null;
+	Event zoomChangedEvent = createZoomChangedEvent(newNativeZoom, processDpiChangeAsynchronous);
 	if (currentDpiChangeEvent != null) {
 		currentDpiChangeEvent.doit = false;
 	}
@@ -5028,13 +5028,15 @@ private void handleMonitorSpecificDpiChange(int newNativeZoom, Rectangle newBoun
 	this.setBoundsInPixels(newBoundsInPixels.x, newBoundsInPixels.y, newBoundsInPixels.width, newBoundsInPixels.height);
 }
 
-Event createZoomChangedEvent(int zoom) {
+Event createZoomChangedEvent(int zoom, boolean asyncExec) {
 	Event event = new Event();
 	event.type = SWT.ZoomChanged;
 	event.widget = this;
 	event.detail = zoom;
 	event.doit = true;
-	event.data = new DPIChangeExecution();
+	DPIChangeExecution dpiChangeExecution = new DPIChangeExecution();
+	dpiChangeExecution.asyncExec = asyncExec;
+	event.data = dpiChangeExecution;
 	return event;
 }