Is there a Flutter overlay widget like a floating Snack Bar?

My Flutter project has a large form with about 20 fields to be filled in by the user. Some of them like name, email etc. are self-explanatory. But a few fields may need additional context to help the user understand their purpose.

I will have a small round button with a question mark icon next to these edit boxes. When the user clicks the icon, I want a rectangular overlay widget to pop up at or near that place. This will have a semi-transparent body, without a header. It will contain one or two lines of context-dependent help text. The user can dispose the widget by simply touching anywhere on the screen. In other words, it will be like a floating SnackBar with two differences: (1) it can be overlaid anywhere on the screen (2) it will not automatically vanish after a time delay.

Can you please name it if such a widget exists, or how the SnackBar can be customized.