What causes clicks to cancel when dragging?

For some buttons, dragging the mouse after you press, but before you release, will cancel the click operation on that button. At first, I thought the ScrollRect was responsible for canceling the click, but after looking at the ScrollRect and Button sources, I can't figure out what's doing it.

So what mechanism is responsible for cancelling clicks when the cursor is dragged?

Thanks

 

That decision is presumably handled at the level of the EventSystem.

The EventSystem has a "drag threshold" property that determines the minimum distance the mouse has to move in order for an action to be considered a "drag" rather than a "click".

But in addition to that, if there are no objects in the object hierarchy being clicked that have defined drag handlers, then I believe the action can count as a click even if the mouse movement exceeds this threshold, as long as the pointerDown and pointerUp events both happen within the bounds of the button.