Hi Veysel,
We will respond back with more detail soon. But, to begin with note that currently you don't receive MouseDoubleClick events on the chart row because the rows are rendered transparent.
There are several ways to make the rows render with a bg color, to begin with try setting SyncRowBackgrounds="true" on the FlexyGantt in XAML, this will make the chart rows render with the same bg color as the grid rows and now you should start receiving MouseDoubleClick events - the e.OriginalSource will now be a control which will have a TaskListControl as an ancestor in the visual tree. The TasksListControl is what represents a row in the FlexyGantt.
Regards,
Praveen
RadiantQ Support