It is this code in src/plugins/openfileslist/openfileslistplugin.cpp which handles that event:
EVT_TREE_ITEM_ACTIVATED(idOpenFilesTree, OpenFilesListPlugin::OnTreeItemActivated)
But there's no such event that will be triggered when the item is single-clicked in wxTreeEvent.
Probably this thread (https://forums.wxwidgets.org/viewtopic.php?t=39267).