[Win32] Fix drag and drop without monitor-specific scaling #2394

To fix drag and drop behavior on multiple monitors with different zooms
when using monitor-specific scaling, a recent change adapted the OS
callbacks for drop operations to run with the required DPI awareness
(PerMonitorV2) in that case. This change did not consider that without
monitor-specific scaling (and particularly when DPI awareness "System"
is used), running with that modified DPI awareness is not correct.

This change adapts the drop callback to only adapt the DPI awareness if
appropriate (i.e., if monitor-specific scaling is enabled).

Fixes #2394

Author: HeikoKlare

bundles/org.eclipse.swt/Eclipse SWT Drag and Drop/win32/org/eclipse/swt/dnd/DropTarget.java

@@ -242,7 +242,7 @@ void createCOMInterfaces() {
 		public long method2(long[] args) {return Release();}
 		@Override
 		public long method3(long[] args) {
-			return Win32DPIUtils.runWithProperDPIAwareness(() -> {
+			return Win32DPIUtils.runWithProperDPIAwareness(getDisplay(), () -> {
 				if (args.length == 5) {
 					return DragEnter(args[0], (int)args[1], (int)args[2], (int)args[3], args[4]);
 				} else {
@@ -252,7 +252,7 @@ public long method3(long[] args) {
 		}
 		@Override
 		public long method4(long[] args) {
-			return Win32DPIUtils.runWithProperDPIAwareness(() -> {
+			return Win32DPIUtils.runWithProperDPIAwareness(getDisplay(), () -> {
 				if (args.length == 4) {
 					return DragOver((int)args[0], (int)args[1], (int)args[2], args[3]);
 				} else {
@@ -264,7 +264,7 @@ public long method4(long[] args) {
 		public long method5(long[] args) {return DragLeave();}
 		@Override
 		public long method6(long[] args) {
-			return Win32DPIUtils.runWithProperDPIAwareness(() -> {
+			return Win32DPIUtils.runWithProperDPIAwareness(getDisplay(), () -> {
 				if (args.length == 5) {
 					return Drop(args[0], (int)args[1], (int)args[2], (int)args[3], args[4]);
 				} else {

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/internal/Win32DPIUtils.java

@@ -19,6 +19,7 @@
 import org.eclipse.swt.graphics.*;
 import org.eclipse.swt.internal.win32.*;
 import org.eclipse.swt.internal.win32.version.*;
+import org.eclipse.swt.widgets.*;
 
 /**
  * This class is used in the win32 implementation only to provide
@@ -68,8 +69,11 @@ public static boolean setDPIAwareness(int desiredDpiAwareness) {
 		return true;
 	}
 
-	public static <T> T runWithProperDPIAwareness(Supplier<T> operation) {
-		// refreshing is only necessary, when monitor specific scaling is active
+	public static <T> T runWithProperDPIAwareness(Display display, Supplier<T> operation) {
+		// only with monitor-specific scaling enabled, the main thread's DPI awareness may be adapted
+		if (!display.isRescalingAtRuntime()) {
+			return operation.get();
+		}
 		long previousDPIAwareness = OS.GetThreadDpiAwarenessContext();
 		try {
 			if (!setDPIAwareness(OS.DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2)) {

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

@@ -1461,7 +1461,7 @@ public Widget findWidget (Widget widget, long id) {
 
 long foregroundIdleProc (long code, long wParam, long lParam) {
 	if (code >= 0) {
-		Runnable processMessages = () -> {
+		Supplier<Boolean> processMessages = () -> {
 			sendPostExternalEventDispatchEvent ();
 			if (runMessagesInIdle) {
 				if (runMessagesInMessageProc) {
@@ -1487,6 +1487,7 @@ long foregroundIdleProc (long code, long wParam, long lParam) {
 			int flags = OS.PM_NOREMOVE | OS.PM_NOYIELD | OS.PM_QS_INPUT;
 			if (!OS.PeekMessage (msg, 0, 0, 0, flags)) wakeThread ();
 			sendPreExternalEventDispatchEvent ();
+			return true;
 		};
 		if (!synchronizer.isMessagesEmpty()) {
 			// Windows hooks will inherit the thread DPI awareness from
@@ -1495,7 +1496,7 @@ long foregroundIdleProc (long code, long wParam, long lParam) {
 			// This requires to reset the thread DPi awareness to make
 			// sure, all UI updates caused by this will be executed
 			// with the correct DPI awareness
-			runWithProperDPIAwareness(processMessages);
+			Win32DPIUtils.runWithProperDPIAwareness(this, processMessages);
 		}
 	}
 	return OS.CallNextHookEx (idleHook, (int)code, wParam, lParam);
@@ -3478,10 +3479,11 @@ long msgFilterProc (long code, long wParam, long lParam) {
 				// This requires to reset the thread DPi awareness to make
 				// sure, all UI updates caused by this will be executed
 				// with the correct DPI awareness
-				runWithProperDPIAwareness(() -> {
+				Win32DPIUtils.runWithProperDPIAwareness(this, () -> {
 					if (!OS.PeekMessage (msg, 0, 0, 0, flags)) {
 						if (runAsyncMessages (false)) wakeThread ();
 					}
+					return true;
 				});
 			}
 			break;
@@ -5414,14 +5416,4 @@ private boolean setMonitorSpecificScaling(boolean activate) {
 	return false;
 }
 
-private void runWithProperDPIAwareness(Runnable operation) {
-	if (isRescalingAtRuntime()) {
-		Win32DPIUtils.runWithProperDPIAwareness(() -> {
-			operation.run();
-			return true;
-		});
-	} else {
-		operation.run();
-	}
-}
 }