Added sanity check to debug parent/child ordering issues (they would generally manifest with an assert/crash in EndFrame bu tthis assert will catch some earlier).

imgui.cpp

@@ -4910,6 +4910,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         // Position child window
         if (flags & ImGuiWindowFlags_ChildWindow)
         {
+            IM_ASSERT(parent_window->Active);
             window->BeginOrderWithinParent = (short)parent_window->DC.ChildWindows.Size;
             parent_window->DC.ChildWindows.push_back(window);
             if (!(flags & ImGuiWindowFlags_Popup) && !window_pos_set_by_api && !window_is_child_tooltip)