Win32 GUI: edit control blur - user-interface

Question

I have a window with an edit control as its child. The control is in focus. Every time I switch to some other application the control loses the focus (blurs). Do I have to keep track of the focused control in order to manually focus back when returning to the app back again? Or am I doing something wrong? I would expect Windows keeps track of such things automatically but who knows?
I'm on Win32 using plain C. Example:
#include <windows.h>
#define NAME "test"
LRESULT CALLBACK WinProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
HWND edit1, edit2;
switch (msg)
{
case WM_CREATE:
edit1 = CreateWindow("edit", "", WS_CHILD|WS_VISIBLE, 0, 0, 200, 50, hWnd, NULL, NULL, NULL);
edit2 = CreateWindow("edit", "", WS_CHILD|WS_VISIBLE, 250, 0, 200, 50, hWnd, NULL, NULL, NULL);
return 0;
case WM_CLOSE:
DestroyWindow(hWnd);
return 0;
case WM_DESTROY:
PostQuitMessage(0);
return 0;
}
return DefWindowProc(hWnd, msg, wParam, lParam);
}
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
WNDCLASS wc;
wc.style = 0;
wc.lpfnWndProc = WinProc;
wc.cbClsExtra = 0;
wc.cbWndExtra = 0;
wc.hInstance = hInstance;
wc.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wc.hCursor = LoadCursor(NULL, IDC_ARROW);
wc.hbrBackground = (HBRUSH)(COLOR_BTNFACE + 1);
wc.lpszMenuName = NAME;
wc.lpszClassName = NAME;
RegisterClass(&wc);
HWND win;
win = CreateWindow(NAME, "test", WS_OVERLAPPEDWINDOW, 0, 0, 500, 500, NULL, NULL, hInstance, NULL);
ShowWindow(win, nCmdShow);
UpdateWindow(win);
MSG msg;
while(GetMessage(&msg, NULL, 0, 0) > 0)
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return msg.wParam;
}

Answer 1

When another application is made active, your window receives the WM_ACTIVATE notification (wParam set to WA_INACTIVE).
When your application is made active again, your window will receive the WM_ACTIVATE notification again, this time with wParam set to WA_ACTIVE or WA_CLICKACTIVE
Since you're not using the dialog manager, you may need to set focus to the edit control yourself when you're activated.
If in doubt, use Spy++ on a dialog window, and watch the message traffic when the app is deactivated and then activated again.*