Caused by rB5929dd7129f6.

Above commit would reset the gizmo highlight on node mouseover.
This would also assert in gizmo_rect_pivot_from_scale_part().

So now, only reset the gizmo when we are not in EVT_GIZMO_UPDATE,
allowing for starting the tweak outside a node and then travelling
'inside' while still preventing to use it over a node when starting a
tweak there.