Currently the preferences have both tweak and drag threshold,
this is confusing because.
- Values under tweak are used to detect click & double click events.
- Drag threshold is _not_ used for what most users would consider dragging, instead it's used specifically for dragging buttons.
This exposes 'drag threshold', using the same value for both.
When checking if a button should be dragged, use the same threshold as for clicking or half the UI-unit size, which is more logical if the user sets the threshold very high.
See: T61260, which initiated this topic.