I am not aware of your current setup on the left sidebar, but I think you display all submenu pages with a tree navigation option as it allows you to display all pages under a currently opened page( this is the new option available for Sitefinity SP2 v.1501). If you want to display all the pages with all subpages and you current selection of expanded collapsed nodes to be persisted while you navigate trough other pages you will have to create a custom navigation that will save the changes of expanded or collapsed nodes trough different pages you navigate to. I have attached a sample script that does the persisting and you can configure it to suit your needs using :
I hope this will help you. Write back if your scenario is different.
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