This new field allows single or double-click (or ENTER) to toggle the expanded/collapsed state of multi-level items via new bits in the XTR'SHOWBUTTON field, defined in XTREE.DEF as:

When enabled, these options may make it easier for the user to operate a multi-level tree, reducing the amount of hand-eye coordination otherwise needed to click precisely on the +/- button.

Burying these options in the XTR'SHOWBUTTON field may be slightly counter-intuitive since they have nothing to do with whether the buttons are visible, but the features are closely related enough that it seemed better than introducing yet another field to the XTRCTL structure.

Note that the double-click option (XTSBF_DBLCLK) probably only makes sense when there is no other action or meaning associated with parent tree items, since it stifles the ability to "choose" the item.