World Machine and Vertex Welds

Sometimes, when I'm using World Machine to make maps for CitiesXL, the image I am working on disappears while I'm building the polygons. My PC is powerful so I guess it's a World Machine issue.
To get around the problem I finish the current polygon and start another knowing I can weld the vertex's together afterwards. When I do this the image shows again.

 Just drag one point in the polygon to another and it will weld. Be careful when you do this that your welding the correct points as un-welding them can sometimes make a bit of a mess. I've had points shoot across the screen a few times.

 It also comes in handy to allow small bodies of water to exist in the map by making multiple polygons and welding them in a way so the body of water will show. See the image above where I have joined three polygons together at certain points to create a lake.