What is the version of Lightroom ?
What is the version of OS ?
Please share screenshots of the issue.
Did you try to hit 'o'?
The version of Lightroom is 6.3 and the operating system is OS X El Capitan whisker version 10.11.2
but it does not change
What should it show? With your setting in the mask it's hardly showing a lot. I can not see a pin in your picture - did you create a filter ?
'o' toggles on a nd off the visibility of the mask (default is red color).
Hit 't' to see the tool bar and you can select the same (ans more options) there.
Grazie mille problema riolto premendo "t"