Right now at work we are making this amazing feature (for a webApp) but i don't know WHEN its correct to use a "mask" Behind the FAB after it gets TAP/CLICKED because in the guide they say nothing about it, all i know is:
- It would be better to improve readability
- I have seen it (The mask) in other google apps
- Pinterest use this behind-mask every time you click on a "PIN"
Below ill link you the Material Design Guide: