The hoverRow is 0 at least when I inspected it from the debugger. And
even though there is the condition self hoverRow > 0 in #balloonText
it gets past that, only to find that self modelIndexFor: self hoverRow
is 0. So either there is a concurrency issue and another process sets
hoverRow, or the hoverRow index (when it was still non-zero) was not
found in viewToModel at the time when the balloon should be displayed.

I'm going to submit zero check in ballonText. Maybe Marcel or anyone
else has a better idea.