Just tested it. I can't leave the Flutter Selector once I open it. Clicking OK or Cancel keeps it. I've tried ESC as well but the selector is still present until I reload the page
There should be a red 'Close' button in the bottom right-hand corner of your screen. I'll add the option to use ESC as well.
I have one (red close button). Flutter on first use seems like a faster workflow than the normal, right click, inspect element, look at the chrome elements, find the tag. Great Job!