message: Begin: Fixed appending into a child window with a second Begin() from a different window stack querying the wrong window for the window->Collapsed test.
diff: @@ -4108,12 +4108,14 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
  else
  flags = window->Flags;
  + // Parent window is latched only on the first call to Begin() of the frame, so further append-calls can be done from a different window stack
  + ImGuiWindow* parent_window = first_begin_of_the_frame ? (!g.CurrentWindowStack.empty() ? g.CurrentWindowStack.back() : NULL) : window->ParentWindow;
  + IM_ASSERT(parent_window != NULL || !(flags & ImGuiWindowFlags_ChildWindow));
  +
  // Add to stack
  - ImGuiWindow* parent_window = !g.CurrentWindowStack.empty() ? g.CurrentWindowStack.back() : NULL;
  g.CurrentWindowStack.push_back(window);
  SetCurrentWindow(window);
  CheckStacksSize(window, true);
  - IM_ASSERT(parent_window != NULL || !(flags & ImGuiWindowFlags_ChildWindow));
  bool window_just_activated_by_user = (window->LastFrameActive < current_frame - 1); // Not using !WasActive because the implicit "Debug" window would always toggle off->on
  if (flags & ImGuiWindowFlags_Popup)
  