mask opacity only makes sense when you have more than one mask... if you want to turn on and off a mask, and you only got one, there are several ways to do this. one way is to add another mask at the size of the layer. clicking twice on the shape tool will do that. set it to "add" place it on top, the set all sub-sequential mask as "subtract" and play with the opacity. another way is use an alpha matte: set a solid layer above the layer. place the mask there. set the bottom layer as alpha-invert matte, set the mask at the top layer as subtract and play with the opacity.
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