There is a setting the Xojo preferences that allows you to use your local copy of the LR, for your specific version of Xojo.
Also, if you know the name of the property or method, e.g., Listbox.LastIndex, you can type that into the Search box of the https://documentation.xojo.com site, i.e., https://documentation.xojo.com/api/deprecated/listbox.html.LastIndex
I hope that helps.