Fill unpainted region in Window in win32 #62

This commit contributes to filling the unpainted region unoccupied by
the menu bar and the client area in a window, including the MenuItem
hover overlay remains in the area after hovering over them as sometimes
MenuItems have their height more than the height of the menu bar,
leading to painting in the unmanaged region - hence, contributing to an
artifact line.

contributes to
#62 and
#127

Author: amartya4256

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

@@ -2303,6 +2303,37 @@ void overpaintMenuBorder () {
 	OS.ReleaseDC (handle, dc);
 }
 
+/**
+ * Fills the remaining area which are not painted by MenuBar and ClientArea
+ * inside the shell window.
+ */
+private void fillUnpaintedRegionInShellWindow() {
+	if (menuBar == null) return;
+	Rectangle clientArea = getClientRectInWindow();
+	Rectangle menuArea = menuBar.getBounds();
+	Rectangle windowBounds = getBoundsInPixels();
+	menuArea.x = menuArea.x - windowBounds.x;
+	menuArea.y = menuArea .y - windowBounds.y;
+	long windowRegion = OS.CreateRectRgn (0, 0, windowBounds.width, windowBounds.height);
+	long menuRegion = OS.CreateRectRgn (menuArea.x, menuArea.y, menuArea.x + menuArea.width, menuArea.y + menuArea.height);
+	long clientRegion = OS.CreateRectRgn (clientArea.x, clientArea.y, clientArea.x + clientArea.width, clientArea.y + clientArea.height);
+	OS.CombineRgn (windowRegion, windowRegion, menuRegion, OS.RGN_DIFF);
+	OS.CombineRgn (windowRegion, windowRegion, clientRegion, OS.RGN_DIFF);
+	OS.DeleteObject (menuRegion);
+	OS.DeleteObject (clientRegion);
+	int dwRop = display.useDarkModeExplorerTheme ? OS.BLACKNESS : OS.PATCOPY;
+	long dc = OS.GetWindowDC (handle);
+	POINT pt = null;
+	pt = new POINT();
+	OS.GetWindowOrgEx(dc, pt);
+	OS.OffsetRgn(windowRegion, -pt.x, -pt.y);
+	OS.SelectClipRgn(dc, windowRegion);
+	OS.OffsetRgn(windowRegion, pt.x, pt.y);
+	OS.PatBlt(dc, 0, 0, windowBounds.width, windowBounds.height, dwRop);
+	OS.DeleteObject (windowRegion);
+	OS.ReleaseDC (handle, dc);
+}
+
 @Override
 long windowProc (long hwnd, int msg, long wParam, long lParam) {
 	if (handle == 0) return 0;
@@ -2347,6 +2378,7 @@ long windowProc (long hwnd, int msg, long wParam, long lParam) {
 		{
 			long ret = super.windowProc (hwnd, msg, wParam, lParam);
 			overpaintMenuBorder();
+			fillUnpaintedRegionInShellWindow();
 			return ret;
 		}
 	}
@@ -2534,6 +2566,16 @@ LRESULT WM_NCHITTEST (long wParam, long lParam) {
 		if (hittest == OS.HTMENU) hittest = OS.HTBORDER;
 		return new LRESULT (hittest);
 	}
+	/*
+	 * In quarter zoom levels, sometimes the MenuItem in the MenuBar has more height
+	 * than the MenuBar, which leads to a gap between the client area and the menu
+	 * bar leaving it unpainted and unmanaged. On hovering over the MenuItem, it
+	 * leaves the gap area painted with remains of the MenuItem hover overlay. The
+	 * event WM_NCHITTEST is sent on hovering over MenuItem and hence the overlay
+	 * remains can be cleaned by calling
+	 * fillUnpaintedRegionBetweenMenuBarAndClientArea on this event.
+	 */
+	fillUnpaintedRegionInShellWindow();
 	return null;
 }