While a track drag hovers within 28px of the track list's top/bottom
edge, the list scrolls that way (40ms timer) so the user can drop onto
off-screen rows. Qt's built-in autoscroll never fires because our
dragMoveEvent accepts the event itself, so this is driven manually; the
drop line refreshes as content scrolls under a stationary cursor.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
