Cure some sick behavior in XBoard Engine Output right-clicks

Cure some sick behavior in XBoard Engine Output right-clicks

When a PV-walk is in progress, the PV is highlighted by selecting it,
and this focuses the attention of the scroll bar on it, so it remains in
view. But after release, that line was kept in view, rather than scrolling
back to the top of the window (where the header line bust be available
for clicking)