Elements (labels or lines) may not be displayed

When the map is panned, in some cases, lines or labels disapear while drawing.
Way to reproduce:
 - load [measure dynamic example](http://jorix.github.io/OL-DynamicMeasure/examples/measure-dynamic.html),
 - activate the measure length control,
 - click on the map to add the first point,
 - click-drag the map to pan so that the first point gets out of visible area,
 - move the mouse cursor -> the line and the labels disapear.