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.