Nav: Modal windows can't be closed with Nav PadCancel. (#787)

imgui.cpp

@@ -3005,6 +3005,7 @@ static void ImGui::NavUpdate()
         else if (g.OpenPopupStack.Size > 0)
         {
             // Close open popup/menu
+            if (!(g.OpenPopupStack.back().Window->Flags & ImGuiWindowFlags_Modal))
                 ClosePopupToLevel(g.OpenPopupStack.Size - 1);
         }
         else if (g.NavLayer != 0)