Cursor and Window position problem

[Valerianel]

I'm trying to create 2 windows and magnet them so they stay close to each other by 10px: the main window and bottom window. The bottom window act as a resizer for main window when user drag it.
Everything is fine with one problem: when main window is moved, the cursor points are not correct anymore. I posted below the pseudo code and a screenshot to show the effect problem.

hwndMain = CreateWindow(L"classMain", L"main", WS_POPUP, 50, 0, 800, 600, NULL, NULL, instanceMain, NULL);
hwndResizer = CreateWindowEx(WS_EX_LAYERED, TEXT("classResizer"), L"", WS_POPUP, 400, 610, 55, 35, hwndMain, NULL, instanceMain, NULL);

resizer proc

case WM_LBUTTONUP: {
		ReleaseCapture();
	} break;

	case WM_LBUTTONDOWN: {
		SetCapture(hwndBottomResizer);
	} break;

	case WM_MOUSEMOVE: {

		if (wParam & MK_LBUTTON) {

			RECT bottomResizerRect, mainWindowRect;
			POINT posBottomResizer;
			
			GetWindowRect(hwndBottomResizer, &bottomResizerRect);
			GetWindowRect(hwndMain, &mainWindowRect);

			posBottomResizer.x = GET_X_LPARAM(lParam); //(int)(short)LOWORD(lParam);
			posBottomResizer.y = GET_Y_LPARAM(lParam); //(int)(short)HIWORD(lParam);
			ClientToScreen(hwndBottomResizer, &posBottomResizer);

			HDWP move = BeginDeferWindowPos(2);
			DeferWindowPos(move, hwndMain, 0, mainWindowRect.left, mainWindowRect.top, mainWindowRect.right - mainWindowRect.left, posBottomResizer.y, 0);
			DeferWindowPos(move, hwndBottomResizer, 0, bottomResizerRect.left, posBottomResizer.y + 10, bottomResizerRect.right - bottomResizerRect.left, 35, 0);
			EndDeferWindowPos(move);

		}

	} break;

main proc

	case WM_NCHITTEST: {

		moveWindows = DefWindowProc(hwnd, message, wParam, lParam);

		RECT rc, brc;

		GetWindowRect(hwnd, &rc);
		GetWindowRect(hwndBottomResizer, &brc);

		POINT pt;
		pt.x = GET_X_LPARAM(lParam);
		pt.y = GET_Y_LPARAM(lParam);
		ScreenToClient(hwnd, &pt);

		HDWP move = BeginDeferWindowPos(1);
		//DeferWindowPos(move, hwndMain, 0, mainWindowRect.left, mainWindowRect.top, mainWindowRect.right - mainWindowRect.left, posBottomResizer.y, 0);
		DeferWindowPos(move, hwndBottomResizer, 0, brc.left, rc.bottom + 10, brc.right - brc.left, 35, 0);
		EndDeferWindowPos(move);

		if (moveWindows == HTCLIENT) {
			return HTCAPTION;
		}
		else {
			return moveWindows;
		}
	} break;

See below gif for effect problem.