SetWindowsHookEx stops receiving WH_MOUSE_LL events when window focus lost - c++

Question

I have an SDL2 program that opens an SDL window. In the main I create a hook for mouse events as
hMSHook = SetWindowsHookEx(WH_MOUSE_LL, MouseProc, NULL, NULL);
while outside of the main I have the custom mouse event handler
HHOOK hMSHook;
int xPosAbsolute = 0;
int yPosAbsolute = 0;
LRESULT CALLBACK MouseProc(int nCode, WPARAM wParam, LPARAM lParam)
{
if (nCode == HC_ACTION)
{
auto &ms = *(const MOUSEHOOKSTRUCT *)lParam;
if (wParam == WM_MOUSEMOVE)
{
xPosAbsolute = ms.pt.x;
yPosAbsolute = ms.pt.y;
}
}
return CallNextHookEx(hMSHook, nCode, wParam, lParam);
}
When the window is in focus, the mouse coordinates xPosAbsolute and yPosAbsolute are properly updated as the mouse moves. However, if another window receives focus, these variables stop being updated.
My question is, how do I make SetWindowsHookEx() continue receiving WH_MOUSE_LL events while my window is not in focus?
EDIT:
Actually, as it turns out, SetWindowsHookEx only stopped receiving event updates when the window focus was lost to another window that was launched as administrator, while the SDL2 window of this program was launched as a regular user. Once the current program is launched as administrator as well, all updates continue to come in (using the above code), even when window focus is lost. So this turned out to be a non-issue.

Answer 1

Focus is not required but keep pumping messages is required.
The following is a mini Win32 window application with low level hook you can refer to. It will receive WM_MOUSEMOVE message in all existing threads running in the same desktop as the calling thread regardless of which window holds the focus.
LRESULT CALLBACK MouseProc(int nCode, WPARAM wParam, LPARAM lParam)
{
if (nCode == HC_ACTION)
{
auto& ms = *(const MOUSEHOOKSTRUCT*)lParam;
if (wParam == WM_MOUSEMOVE)
{
OutputDebugString(L"\n WM_MOUSEMOVE \n");
OutputDebugStringA(to_string(counter++).c_str());
}
}
return CallNextHookEx(NULL, nCode, wParam, lParam);
}
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
return DefWindowProc(hWnd, message, wParam, lParam);
}
int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
_In_opt_ HINSTANCE hPrevInstance,
_In_ LPWSTR lpCmdLine,
_In_ int nCmdShow)
{
HHOOK hMSHook = SetWindowsHookEx(WH_MOUSE_LL, MouseProc, NULL, NULL);
WNDCLASSEXW wcex = {0};
wcex.cbSize = sizeof(WNDCLASSEX);
wcex.lpfnWndProc = WndProc;
wcex.hInstance = hInstance;
wcex.lpszClassName = szWindowClass;
RegisterClassExW(&wcex);
HWND hWnd = CreateWindowW(szWindowClass, L"test", WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, nullptr, nullptr, hInstance, nullptr);
ShowWindow(hWnd, nCmdShow);
UpdateWindow(hWnd);
MSG msg;
// Main message loop:
while (GetMessage(&msg, nullptr, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return (int) msg.wParam;
}