WinAPI client area not being redrawn after SetWindowPos() - winapi

Question

I am trying to resize the window of another application (an ipoker client window), using this code:
SetWindowPos(hwnd, HWND_NOTOPMOST, x, y, width, height, SWP_ASYNCWINDOWPOS | SWP_NOACTIVATE | SWP_NOOWNERZORDER | SWP_NOZORDER);
Now this resizes the window properly, but the client area is not getting redrawn (meaning its just clipped to fit the new window area). This is how it should look:
how the window looks properly
But this is how it actually looks: client area is just being clipped
So I've inspected the messages the window receives upon resizing it by mouse (which works fine) with Spy++ and tried to just send all of them to the window using SendMessage(..), but without any luck as well.
WINDOWPOS wpos;
wpos.hwnd = hWnd;
wpos.x = x;
wpos.y = y;
wpos.cx = nWidth;
wpos.cy = nHeight;
wpos.flags = SWP_NOZORDER | SWP_NOACTIVATE | SWP_NOMOVE | 1000;
NCCALCSIZE_PARAMS params;
params.lppos = &wpos;
params.rgrc[0] = { x, y, width, height};
params.rgrc[1] = rcWindow; // rect of the window before resize
params.rgrc[2] = rcClient; // rect of the client before resize
SendMessage(hWnd, WM_WINDOWPOSCHANGED, 0, LPARAM(&wpos));
wpos.flags = SWP_NOZORDER | SWP_NOACTIVATE;
SendMessage(hwnd, WM_SIZE, SIZE_RESTORED, MAKELPARAM(nWidth, nHeight));
SendMessage(hwnd, WM_WINDOWPOSCHANGING, 0, LPARAM(&wpos));
wpos.flags = SWP_NOZORDER | SWP_NOACTIVATE | SWP_NOMOVE | 1800;
SendMessage(hwnd, WM_NCCALCSIZE, WPARAM(TRUE), LPARAM(&params));
Do you have any idea why all of this is not working? Is there no way around subclassing the window (I dont have any experience with that..)? I've also tried to resize the window using placemint (a window positioning tool), but it turns out to have the same problem. The only way to force it to redraw I've found is resize it manually.