WinAPI timer callback not called while child window is in focus

Question

I'm working on a 3D editor app using Direct3D and WinAPI. I create a main window, which has a child window that takes up a portion of the main window's client area. This is used by D3D as the render target. I then also create a separate window with CreateWindow, which is built the same way as the main window (i.e a "main window" and an internal child used as a render target), and I make this window a child of the main application window (to ensure that they are minimized/restored/closed together).

The D3D rendering is executed by the render target child windows processing their WM_PAINT messages. To reduce unnecessary overhead, I set the window procedures to only render on WM_PAINT if GetForegroundWindow and GetFocus match the respective window handles. In other words, I only want a window's rendering to be refreshed if it's on top and is focused.

This is my main message loop:

HWND mainWnd;
HWND mainRenderWnd;
HWND childWnd;
HWND childRenderWnd;

// ...

MSG msg = {};
while (WM_QUIT != msg.message)
{
    if (PeekMessage(&msg, nullptr, 0, 0, PM_REMOVE))
    {
        if (!TranslateAccelerator(mainWnd, accel, &msg))
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }
    else
    {
        // Run non-UI code...
    }
}

When the main window gets WM_SETFOCUS, I have it set the focus to its render target child window, since I'll want to process inputs there (e.g camera controls):

// ...
LRESULT CALLBACK MainWndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    switch(uMsg)
    {
        //...
        case WM_SETFOCUS:
        {
            SetFocus(mainRenderWnd);
            return 0;
        }
        //...
    }
    return DefWindowProc(hWnd, uMsg, wParam, lParam);
}

I do the same in the window procedure for childWnd, setting the focus to childRenderWnd. This window is opened and closed by the user, i.e at any one time it may or may not exist, but when it does and is not minimized, it needs to be the foreground window with focus. Also, to control the framerate for the child window, I use a Timer to refresh it:

static constexpr UINT_PTR RENDER_TIMER_ID = (UINT_PTR)0x200;
void TimerCallback(HWND Arg1, UINT Arg2, UINT_PTR Arg3, DWORD Arg4)
{
    if (IsIconic(childWnd) || !(GetFocus() == childRenderWnd))
        return;

    // Invalidate the render area to make sure it gets redrawn
    InvalidateRect(childRenderWnd, nullptr, false);
}

// ...

SetTimer(childWnd, RENDER_TIMER_ID, 16, (TIMERPROC)TimerCallback);

With all this set up, mainWnd and mainRenderWnd seem to work just fine. However, childRenderWnd refuses to have anything rendered to it when it is in the foreground and in focus. While debugging, I found that while this is the case, the timer callback never gets executed, nor does a WM_TIMER message get dispatched to the child window.

On the other hand, the moment I deliberately move focus out of the child window and onto the main window (while keeping both open), the timer message gets sent, and the callback is executed. Another problem is that when I minimize the app while both windows are open, and then restore them both, the render target of neither of the windows is refreshed. Instead, it seems like the focus got "flipped", as I have to click on my child window first, then my main window, and that makes it refresh properly (while the child still refuses to render anything).

What am I missing? I searched for others having problem, e.g an incorrect message pump setup blocking WM_TIMER, but nothing seems to explain what's going on here.

Thanks in advance for the help!

Answer 1

IInspectable and Raymond Chen's comments have helped lead me to the answer. I tried to reproduce the error in a minimal app and finally came upon the source of my trouble. Originally, the main window would just call InvalidateRect in the message loop if there were no messages to be dispatched, effectively redrawing itself every chance it got. This originally did not seem to cause any harm, the scene was rendered just fine, and the window responded to inputs. Once I introduced a second window, however, it must have flooded the message loop with paint messages, making it impossible for any timer messages to get through.

The solution, quite simply, was to give a timer to both windows to set the rate at which they would refresh their D3D render targets. Coupled with focus checks, I can now easily alternate between both windows, with only one refreshing itself at any given time.

The question remains whether the WinAPI timer system is the best choice. My bad code aside, people have mentioned that the timer's messages are low-priority, which might make it a poor choice for framerate control in the long run.

EDIT: I ended up going with IInspectable's suggestion to use a while loop within the main message loop to process to dispatch all messages, and once those are processed, I perform a frame update. This allows me to consistently update all windows and control the frame rate without risking the issue of window messages being stuck.