White Flicker with CombineRgn

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C++

Win32

regions

It seems the flickering is generated by the CombineRgn function, but I really have no idea why this happens, since i've never used regions that much I'm possibly missing some knowledge on the matter.

Some events in the program triggers the addition of little rectangles to the main region, here's the code that handles that:

HRGN ActualRegion = CreateRectRgn(0, 0, 0, 0);
GetWindowRgn(hwnd, ActualRegion);
HRGN AddedRect = CreateRectRgn(//long code that creates a rectangle)
CombineRgn(ActualRegion, ActualRegion, AddedRect, RGN_OR);

SetWindowRgn(hwnd, ActualRegion, FALSE);
InvalidateRect(hwnd, NULL, FALSE);

White Flickering appears only after the invalidation if new regions where combined to the main one.


Here's how I'm implementing double buffering in WM_PAINT:
PLEASE NOTE that on creation i'm enabling the DWM blur behind function with an invalid region (different from the Main one) which means that everything painted with BLACK_BRUSH will result in a 100% "invisible" portion of the program

RECT r;	GetClientRect(hwnd, &r);

		PAINTSTRUCT ps; HDC hdc = BeginPaint(hwnd, &ps);
		HDC MemDc = CreateCompatibleDC(hdc);
		HBITMAP hBmp = CreateCompatibleBitmap(hdc, r.right, r.bottom);
		HBITMAP hOld = (HBITMAP)SelectObject(MemDc, hBmp);

		//Making sure this dc is filled with "invisible" pixels to display
		SelectObject(MemDc, GetStockObject(BLACK_BRUSH));
		Rectangle(MemDc, //arbitrary values that matches the entire screen);

		BitBlt(hdc, 0, 0, GetSystemMetrics(SM_CXSCREEN), GetSystemMetrics(SM_CYSCREEN), MemDc, 0, 0, 		SRCCOPY);

		//clean-up
		SelectObject(MemDc, hOld);
		DeleteObject(hBmp);
		DeleteDC(MemDc);
		EndPaint(hwnd, &ps);

WM_ERASEBKGND obviously returns TRUE without further handling, the WNDCLASSEX instance of the window has a default BLACK_BRUSH as the hbrBackground field.
Is it something that i'm possibly missing ? I can't really understand what may be causing white flickering in this scenario