Thumbnail file drag kicked off after only 10px of movement, which made
it too easy to start a drag when the user meant to rubber-band select
or just click-and-micro-wobble. Bumped to 30px so the gate matches the
rubber band's own threshold in `_maybe_start_rb`.
behavior change: tiny mouse movement on a thumbnail no longer starts a
file drag; you now need to drag ~30px before the OS drag kicks in.
pax
3b3de35689