Text Boxes in the module's text field

I am creating a module for a game, but every time I move a piece from the game palette to the map (setting up a scenario) I get these boxes marching across the text field. I have noticed this also when playing a couple of modules I have made. Clearly I am doing something wrong (it doesn’t appear to be Vassal’s fault) as I don’t experience this with other people’s modules.
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