Demo: fixed multi-selection tree nodes demo to not replace selection when clicking on single-item that's already part of selection (#581)

imgui_demo.cpp

@@ -276,7 +276,7 @@ void ImGui::ShowTestWindow(bool* p_open)
                     // Update selection state. Process outside of tree loop to avoid visual inconsistencies during the clicking-frame.
                     if (ImGui::GetIO().KeyCtrl)
                         selection_mask ^= (1 << node_clicked);          // CTRL+click to toggle
-                    else
+                    else if (!(selection_mask & (1 << node_clicked)))   // If there is already a selection don't replace we clicked node is part of it
                         selection_mask = (1 << node_clicked);           // Click to single-select
                 }
                 ImGui::TreePop();