Calling hou.UIEvent.reason() returns one of the following values.
Quick mouse click without dragging.
Left mouse button pressed (mouse down)
Mouse dragged with the left mouse button down.
Left mouse button released (mouse up).
Mouse pointer hovered over something in the interface. In a viewer state, this is the reason associated with mouse moves.
Event generated as a change of values in hou.UIEvent
Event generated when a UI element was assigned a different value.
Event generated when a slider or scrollbar has changed.
Event was likely explicitly generated.