[win32] Fix DnD across multiple monitors

This commit addresses issues, when DnD across multiple monitors with
monitors specific scaling is used. If the monitors have different zoom
dragging into a table or tree on the second monitor will detect the wrong
destination items depending on the zoom. To address this the zoom should
always be inherited from the underlying control if available.

Author: akoch-yatta

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

@@ -14,6 +14,7 @@
 package org.eclipse.swt.dnd;
 
 import org.eclipse.swt.*;
+import org.eclipse.swt.graphics.*;
 import org.eclipse.swt.internal.*;
 import org.eclipse.swt.internal.ole.win32.*;
 import org.eclipse.swt.internal.win32.*;
@@ -281,16 +282,14 @@ int DragEnter_64(long pDataObject, int grfKeyState, long pt, long pdwEffect) {
 }
 
 int DragEnter(long pDataObject, int grfKeyState, int pt_x, int pt_y, long pdwEffect) {
-	int zoom = DPIUtil.getZoomForAutoscaleProperty(nativeZoom);
-	pt_x = DPIUtil.scaleDown(pt_x, zoom);// To Points
-	pt_y = DPIUtil.scaleDown(pt_y, zoom);// To Points
+	Point location = convertLocation(pt_x, pt_y);
 	selectedDataType = null;
 	selectedOperation = DND.DROP_NONE;
 	if (iDataObject != null) iDataObject.Release();
 	iDataObject = null;
 
 	DNDEvent event = new DNDEvent();
-	if (!setEventData(event, pDataObject, grfKeyState, pt_x, pt_y, pdwEffect)) {
+	if (!setEventData(event, pDataObject, grfKeyState, location.x, location.y, pdwEffect)) {
 		OS.MoveMemory(pdwEffect, new int[] {COM.DROPEFFECT_NONE}, 4);
 		return COM.S_FALSE;
 	}
@@ -349,14 +348,12 @@ int DragOver_64(int grfKeyState, long pt, long pdwEffect) {
 }
 
 int DragOver(int grfKeyState, int pt_x, int pt_y, long pdwEffect) {
-	int zoom = DPIUtil.getZoomForAutoscaleProperty(nativeZoom);
-	pt_x = DPIUtil.scaleDown(pt_x, zoom);// To Points
-	pt_y = DPIUtil.scaleDown(pt_y, zoom);// To Points
+	Point location = convertLocation(pt_x, pt_y);
 	if (iDataObject == null) return COM.S_FALSE;
 	int oldKeyOperation = keyOperation;
 
 	DNDEvent event = new DNDEvent();
-	if (!setEventData(event, iDataObject.getAddress(), grfKeyState, pt_x, pt_y, pdwEffect)) {
+	if (!setEventData(event, iDataObject.getAddress(), grfKeyState, location.x, location.y, pdwEffect)) {
 		keyOperation = -1;
 		OS.MoveMemory(pdwEffect, new int[] {COM.DROPEFFECT_NONE}, 4);
 		return COM.S_FALSE;
@@ -403,23 +400,38 @@ int Drop_64(long pDataObject, int grfKeyState, long pt, long pdwEffect) {
 	return Drop(pDataObject, grfKeyState, point.x, point.y, pdwEffect);
 }
 
+private Point convertLocation(int xInPixels, int yInPixels) {
+	if (this.control == null) {
+		// If there is no control for context, the behavior remains as before
+		int zoom = DPIUtil.getZoomForAutoscaleProperty(this.nativeZoom);
+		return DPIUtil.scaleDown(new Point(xInPixels, yInPixels), zoom);
+	}
+	int zoom = DPIUtil.getZoomForAutoscaleProperty(this.control.nativeZoom);
+	// There is no API to convert absolute values in pixels to display relative
+	// points. Therefor, the display relative pixel values are converted to control
+	// relative pixel values via Windows API. These values can be scaled down to points
+	POINT pt = new POINT ();
+	pt.x = xInPixels;  pt.y = yInPixels;
+	OS.ScreenToClient (this.control.handle, pt);
+	Point p = DPIUtil.scaleDown(new Point (pt.x, pt.y), zoom);
+	return this.control.toDisplay(p);
+}
+
 int Drop(long pDataObject, int grfKeyState, int pt_x, int pt_y, long pdwEffect) {
 	try {
-		int zoom = DPIUtil.getZoomForAutoscaleProperty(nativeZoom);
-		pt_x = DPIUtil.scaleDown(pt_x, zoom);// To Points
-		pt_y = DPIUtil.scaleDown(pt_y, zoom);// To Points
+		Point location = convertLocation(pt_x, pt_y);
 		DNDEvent event = new DNDEvent();
 		event.widget = this;
 		event.time = OS.GetMessageTime();
 		if (dropEffect != null) {
-			event.item = dropEffect.getItem(pt_x, pt_y);
+			event.item = dropEffect.getItem(location.x, location.y);
 		}
 		event.detail = DND.DROP_NONE;
 		notifyListeners(DND.DragLeave, event);
 		refresh();
 
 		event = new DNDEvent();
-		if (!setEventData(event, pDataObject, grfKeyState, pt_x, pt_y, pdwEffect)) {
+		if (!setEventData(event, pDataObject, grfKeyState, location.x, location.y, pdwEffect)) {
 			keyOperation = -1;
 			OS.MoveMemory(pdwEffect, new int[] {COM.DROPEFFECT_NONE}, 4);
 			return COM.S_FALSE;