MenuItem not aligned correctly during DPI change

When switched to Dark Theme, using gc to draw menu item, moving the
responsibility from OS due to wrong scaling of menu bar vertically.
Also changing the text color from a grayish tone to white.
Also when ALT key is pressed mnemonics are underlined and work as a toggle, the behavior which was missing from dark theme previously.

Author: ShahzaibIbrahim

bundles/org.eclipse.swt/Eclipse SWT PI/win32/org/eclipse/swt/internal/win32/OS.java

@@ -897,6 +897,7 @@ public class OS extends C {
 	public static final int MFS_CHECKED = 0x8;
 	public static final int MFS_DISABLED = 0x3;
 	public static final int MFS_GRAYED = 0x3;
+	public static final int MFT_OWNERDRAW = 0x100;
 	public static final int MFT_RADIOCHECK = 0x200;
 	public static final int MFT_RIGHTJUSTIFY = 0x4000;
 	public static final int MFT_RIGHTORDER = 0x2000;
@@ -1019,6 +1020,8 @@ public class OS extends C {
 	public static final int OBJ_PEN = 0x1;
 	public static final int OBM_CHECKBOXES = 0x7ff7;
 	public static final int ODS_SELECTED = 0x1;
+	public static final int ODS_NOACCEL = 0x0100;
+	public static final int ODS_INACTIVE = 0x80;
 	public static final int ODT_MENU = 0x1;
 	public static final int OIC_BANG = 0x7F03;
 	public static final int OIC_HAND = 0x7F01;

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

@@ -358,7 +358,7 @@ void createItem (MenuItem item, int index) {
 	info.fMask = OS.MIIM_ID | OS.MIIM_TYPE | OS.MIIM_DATA;
 	info.wID = item.id;
 	info.dwItemData = item.id;
-	info.fType = item.widgetStyle ();
+	info.fType = (style & SWT.BAR) != 0 && needsMenuCallback() ? OS.MFT_OWNERDRAW :  item.widgetStyle ();
 	info.dwTypeData = pszText;
 	boolean success = OS.InsertMenuItem (handle, index, true, info);
 	if (pszText != 0) OS.HeapFree (hHeap, 0, pszText);

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

@@ -47,7 +47,12 @@ public class MenuItem extends Item {
 	/* Image margin. */
 	final static int MARGIN_WIDTH = 1;
 	final static int MARGIN_HEIGHT = 1;
-
+	private final static int LEFT_TEXT_MARGIN = 7;
+	private final static int IMAGE_TEXT_GAP = 3;
+	// There is a weird behavior in the Windows API with menus in OWENERDRAW mode that the returned
+	// value in wmMeasureChild is increased by a fixed value (in points) when wmDrawChild is called
+	// This static is used to mitigate this increase
+	private final static int WINDOWS_OVERHEAD = 6;
 	static {
 		DPIZoomChangeRegistry.registerHandler(MenuItem::handleDPIChange, MenuItem.class);
 	}
@@ -1121,10 +1126,32 @@ LRESULT wmCommandChild (long wParam, long lParam) {
 	return null;
 }
 
-LRESULT wmDrawChild (long wParam, long lParam) {
-	DRAWITEMSTRUCT struct = new DRAWITEMSTRUCT ();
-	OS.MoveMemory (struct, lParam, DRAWITEMSTRUCT.sizeof);
-	if (image != null) {
+@Override
+GC createNewGC(long hDC, GCData data) {
+	if (getDisplay().isRescalingAtRuntime()) {
+		return super.createNewGC(hDC, data);
+	} else {
+		data.nativeZoom = getMonitorZoom();
+		return GC.win32_new(hDC, data);
+	}
+}
+
+private int getMonitorZoom() {
+	return getMenu().getShell().getMonitor().zoom;
+}
+
+private int getMenuZoom() {
+	if (getDisplay().isRescalingAtRuntime()) {
+		return super.getZoom();
+	} else {
+		return DPIUtil.getZoomForAutoscaleProperty(getMonitorZoom());
+	}
+}
+
+LRESULT wmDrawChild(long wParam, long lParam) {
+	DRAWITEMSTRUCT struct = new DRAWITEMSTRUCT();
+	OS.MoveMemory(struct, lParam, DRAWITEMSTRUCT.sizeof);
+	if ((text != null || image != null)) {
 		GCData data = new GCData();
 		data.device = display;
 		GC gc = createNewGC(struct.hDC, data);
@@ -1134,14 +1161,50 @@ LRESULT wmDrawChild (long wParam, long lParam) {
 		* coordinate.  The fix is to ignore this value when
 		* the item is in a menu bar.
 		*/
-		int x = (parent.style & SWT.BAR) != 0 ? MARGIN_WIDTH * 2 : struct.left;
-		Image image = getEnabled () ? this.image : new Image (display, this.image, SWT.IMAGE_DISABLE);
-		int zoom = getZoom();
-		gc.drawImage (image, DPIUtil.scaleDown(x, zoom), DPIUtil.scaleDown(struct.top + MARGIN_HEIGHT, zoom));
-		if (this.image != image) image.dispose ();
-		gc.dispose ();
+		int x = (parent.style & SWT.BAR) == 0 ? MARGIN_WIDTH * 2 : struct.left;
+		int zoom = getMenuZoom();
+		Rectangle menuItemArea = null;
+		if (text != null) {
+			this.getParent().redraw();
+			int flags = SWT.DRAW_DELIMITER;
+			boolean isInactive = ((struct.itemState & OS.ODS_INACTIVE) != 0);
+			boolean isSelected = ((struct.itemState & OS.ODS_SELECTED) != 0);
+			boolean isNoAccel = ((struct.itemState & OS.ODS_NOACCEL) != 0);
+
+			String drawnText = "";
+			if(isNoAccel) {
+				drawnText = this.text.replace("&", "");
+			} else {
+				drawnText = this.text;
+				flags |= SWT.DRAW_MNEMONIC;
+			}
+			Rectangle menuItemBounds = this.getBounds();
+
+			int fillMenuWidth =  DPIUtil.scaleDown(menuItemBounds.width, zoom);
+			int fillMenuHeight = DPIUtil.scaleDown(menuItemBounds.height, zoom);
+			menuItemArea = new Rectangle(DPIUtil.scaleDown(x, zoom), DPIUtil.scaleDown(struct.top, zoom), fillMenuWidth, fillMenuHeight);
+
+			gc.setForeground(isInactive ? display.getSystemColor(SWT.COLOR_GRAY) : display.getSystemColor(SWT.COLOR_WHITE));
+			gc.setBackground(isSelected ? display.getSystemColor(SWT.COLOR_DARK_GRAY) : parent.getBackground());
+			gc.fillRectangle(menuItemArea);
+
+			int xPositionText = LEFT_TEXT_MARGIN + DPIUtil.scaleDown(x, zoom) + (this.image != null ? this.image.getBounds().width + IMAGE_TEXT_GAP : 0);
+			int yPositionText = DPIUtil.scaleDown(struct.top, zoom) + MARGIN_HEIGHT;
+			gc.drawText(drawnText, xPositionText, yPositionText, flags);
+		}
+		if (image != null) {
+			Image image = getEnabled() ? this.image : new Image(display, this.image, SWT.IMAGE_DISABLE);
+			int gap = (menuItemArea.height - image.getBounds().height) / 2;
+			gc.drawImage(image, LEFT_TEXT_MARGIN + DPIUtil.scaleDown(x, zoom), gap + DPIUtil.scaleDown(struct.top, zoom));
+			if (this.image != image) {
+				image.dispose();
+			}
+		}
+		gc.dispose();
+	}
+	if (parent.foreground != -1) {
+		OS.SetTextColor(struct.hDC, parent.foreground);
 	}
-	if (parent.foreground != -1) OS.SetTextColor (struct.hDC, parent.foreground);
 	return null;
 }
 
@@ -1151,17 +1214,19 @@ LRESULT wmMeasureChild (long wParam, long lParam) {
 
 	if ((parent.style & SWT.BAR) != 0) {
 		if (parent.needsMenuCallback()) {
+			Point point = calculateRenderedTextSize();
+			int menuZoom = getDisplay().isRescalingAtRuntime() ? super.getZoom() : getMonitorZoom();
+			struct.itemHeight = DPIUtil.scaleUp(point.y, menuZoom);
 			/*
 			 * Weirdness in Windows. Setting `HBMMENU_CALLBACK` causes
 			 * item sizes to mean something else. It seems that it is
 			 * the size of left margin before the text. At the same time,
 			 * if menu item has a mnemonic, it's always drawn at a fixed
 			 * position. I have tested on Win7, Win8.1, Win10 and found
 			 * that value of 5 works well in matching text to mnemonic.
-			 * NOTE: autoScaleUpUsingNativeDPI() is used to avoid problems
-			 * with applications that disable automatic scaling.
 			 */
-			struct.itemWidth = DPIUtil.scaleUp(5, nativeZoom);
+			int horizontalSpaceImage = this.image != null ? this.image.getBounds().width + IMAGE_TEXT_GAP: 0;
+			struct.itemWidth = DPIUtil.scaleUp(LEFT_TEXT_MARGIN + point.x - WINDOWS_OVERHEAD + horizontalSpaceImage, menuZoom);
 			OS.MoveMemory (lParam, struct, MEASUREITEMSTRUCT.sizeof);
 			return null;
 		}
@@ -1205,6 +1270,32 @@ LRESULT wmMeasureChild (long wParam, long lParam) {
 	return null;
 }
 
+private Point calculateRenderedTextSize() {
+	GC gc = new GC(this.getMenu().getShell());
+	String textWithoutMnemonicCharacter = getText().replace("&", "");
+	Point points = gc.textExtent(textWithoutMnemonicCharacter);
+	gc.dispose();
+
+	if (!getDisplay().isRescalingAtRuntime()) {
+		int primaryMonitorZoom = this.getDisplay().getDeviceZoom();
+		int adjustedPrimaryMonitorZoom = DPIUtil.getZoomForAutoscaleProperty(primaryMonitorZoom);
+		if (primaryMonitorZoom != adjustedPrimaryMonitorZoom) {
+			// Windows will use a font matching the native primary monitor zoom for calculating the size in pixels,
+			// GC will use the native primary monitor zoom to scale down from pixels to points in this scenario
+			// Therefore we need to make sure adjust the points as if it would have been scaled down by the
+			// native primary monitor zoom.
+			// Example:
+			// Primary monitor on 150% with int200: native zoom 150%, adjusted zoom 100%
+			// Pixel height of font in this example is 15px
+			// GC calculated height of 15px, scales down with adjusted zoom of 100% and returns 15pt -> should be 10pt
+			// this calculation is corrected by the following line
+			// This is the only place, where the GC needs to use the native zoom to do that, therefore it is fixed only here
+			points = DPIUtil.scaleDown(DPIUtil.scaleUp(points, adjustedPrimaryMonitorZoom), primaryMonitorZoom);
+		}
+	}
+	return points;
+}
+
 private static final class MenuItemToolTip extends ToolTip {
 
 	public MenuItemToolTip(Shell parent) {