This is a feature of Window Explorer in single-click mode, which allows to change the selection while pointing to item using the mouse cursor. The implementation is somewhat tricky for atleast the ExoTreeView. In ExoIconView this can be done using the prelit_item.
Created attachment 493 Patch for ExoIconView and ExoTreeView First attempt to implement this behavior for ExoIconView and ExoTreeView, using another property "single-click-timeout", which is the delay after which the prelited item will be selected automatically, or 0 to disable.
Committed with revision 20828. 2006-04-09 Benedikt Meurer <benny@xfce.org> * exo/exo-icon-view.{c,h}, exo/exo-tree-view.{c,h}, exo/exo.symbols: Add "single-click-timeout" properties to ExoIconView and ExoTreeView, which control the delay after which the item under the mouse pointer is automatically selected. Bug #1509. * docs/reference/: Update API documentation.