user32: Check WS_CHILD style in IsChild function.

dlls/user32/tests/win.c

@@ -3675,6 +3675,19 @@ static void test_SetParent(void)
     ok(SetParent(sibling, parent) != 0, "SetParent should not fail\n");
     ok(GetMenu(sibling) == hMenu, "SetParent should not remove menu\n");
 
+    ok(SetParent(parent, desktop) != 0, "SetParent should not fail\n");
+    ok(SetParent(child4, child3) != 0, "SetParent should not fail\n");
+    ok(SetParent(child3, child2) != 0, "SetParent should not fail\n");
+    ok(SetParent(child2, child1) != 0, "SetParent should not fail\n");
+    ok(!IsChild(child3, child4), "wrong parent/child %p/%p\n", child3, child4);
+    SetWindowLongW(child4, GWL_STYLE, WS_CHILD);
+    ok(IsChild(child3, child4), "wrong parent/child %p/%p\n", child3, child4);
+    ok(IsChild(child2, child4), "wrong parent/child %p/%p\n", child2, child4);
+    ok(!IsChild(child1, child4), "wrong parent/child %p/%p\n", child1, child4);
+    SetWindowLongW(child2, GWL_STYLE, WS_CHILD);
+    ok(IsChild(child1, child4), "wrong parent/child %p/%p\n", child1, child4);
+    ok(IsChild(parent, child4), "wrong parent/child %p/%p\n", parent, child4);
+
     ok(DestroyWindow(parent), "DestroyWindow() failed\n");
 
     ok(!IsWindow(parent), "parent still exists\n");

dlls/user32/win.c

@@ -3043,14 +3043,22 @@ HWND WINAPI SetParent( HWND hwnd, HWND parent )
  */
 BOOL WINAPI IsChild( HWND parent, HWND child )
 {
-    HWND *list = list_window_parents( child );
+    HWND *list;
     int i;
-    BOOL ret;
+    BOOL ret = FALSE;
 
-    if (!list) return FALSE;
+    if (!(GetWindowLongW( child, GWL_STYLE ) & WS_CHILD)) return FALSE;
+    if (!(list = list_window_parents( child ))) return FALSE;
     parent = WIN_GetFullHandle( parent );
-    for (i = 0; list[i]; i++) if (list[i] == parent) break;
-    ret = list[i] && list[i+1];
+    for (i = 0; list[i]; i++)
+    {
+        if (list[i] == parent)
+        {
+            ret = list[i] && list[i+1];
+            break;
+        }
+        if (!(GetWindowLongW( list[i], GWL_STYLE ) & WS_CHILD)) break;
+    }
     HeapFree( GetProcessHeap(), 0, list );
     return ret;
 }