help with the source tree

Hi guys. I would like to try to solve one bug in ardour (just try,
really. I'm not a master programmer or anything like that), and I have
no idea about where to start looking, since the code is huge.

It is this bug http://tracker.ardour.org/view.php?id=6270 and it is
about notes that you input in midi tracks, in write mode, by clicking
and dragging the mouse, after a tempo change, coming with the wrong
duration.

Could someone point to the files / classes / functions related to this?