video/out/w32_common: support --wid=0

Similar to X11, this makes mpv attach to the desktop wallpaper window
(make the system create one if it does not yet exist) as parent,
which makes mpv display as desktop wallpaper behind desktop icons.

Lika an actual wallpaper, mpv will not receive any keyboard or mouse
input in this mode. It can still be controlled through terminal and IPC.

Author: na-na-hi

DOCS/interface-changes/win32-wid0.txt

@@ -0,0 +1 @@
+add `--wid=0` support for win32

video/out/w32_common.c

@@ -100,6 +100,7 @@ struct vo_w32_state {
     HWND parent; // 0 normally, set in embedding mode
     HHOOK parent_win_hook;
     HWINEVENTHOOK parent_evt_hook;
+    bool embed_desktop;
 
     struct menu_ctx *menu_ctx;
 
@@ -2025,6 +2026,36 @@ static void w32_api_load(struct vo_w32_state *w32)
                 (void *)GetProcAddress(uxtheme_dll, MAKEINTRESOURCEA(135));
 }
 
+static BOOL CALLBACK EnumWindowsProc(HWND hwnd, LPARAM lparam)
+{
+    HWND *workerw = (HWND *)lparam;
+    HWND defview = FindWindowExW(hwnd, NULL, L"SHELLDLL_DefView", NULL);
+    if (defview) {
+        *workerw = FindWindowExW(NULL, hwnd, L"WorkerW", NULL);
+        return FALSE;
+    }
+    return TRUE;
+}
+
+// Find the "WorkerW" HWND to attach to, which is the desktop wallpaper HWND.
+static HWND get_workerw_hwnd(void)
+{
+    HWND progman = FindWindowW(L"Progman", NULL);
+    // 24H2 or later
+    HWND workerw = FindWindowExW(progman, NULL, L"WorkerW", NULL);
+    // Previous Windows versions
+    if (!workerw)
+        EnumWindows(EnumWindowsProc, (LPARAM)&workerw);
+    return workerw;
+}
+
+// Enter the "raised desktop" mode with wallpaper and icons on separate HWNDs.
+static void set_raised_desktop(void)
+{
+    HWND progman = FindWindowW(L"Progman", NULL);
+    SendMessageTimeout(progman, 0x052c, 0xd, 1, SMTO_NORMAL, 1000, NULL);
+}
+
 static MP_THREAD_VOID gui_thread(void *ptr)
 {
     struct vo_w32_state *w32 = ptr;
@@ -2035,8 +2066,13 @@ static MP_THREAD_VOID gui_thread(void *ptr)
 
     w32_api_load(w32);
 
-    if (w32->opts->WinID >= 0)
+    if (w32->opts->WinID == 0) {
+        set_raised_desktop();
+        w32->parent = get_workerw_hwnd();
+        w32->embed_desktop = true;
+    } else if (w32->opts->WinID > 0) {
         w32->parent = (HWND)(intptr_t)(w32->opts->WinID);
+    }
 
     ATOM cls = get_window_class();
     if (w32->parent) {
@@ -2148,6 +2184,10 @@ static MP_THREAD_VOID gui_thread(void *ptr)
         ITaskbarList3_Release(w32->taskbar_list3);
     if (ole_ok)
         OleUninitialize();
+    // Some Windows versions do not redraw the desktop wallpaper when mpv exits,
+    // so force a redraw here.
+    if (w32->embed_desktop)
+        set_raised_desktop();
     SetThreadExecutionState(ES_CONTINUOUS);
     MP_THREAD_RETURN();
 }