I've tried 'Checked = "true"', 'CheckedState = "true"', 'Selected = "true"'. None of these seem to do anything...
Count me too!
I posted this in prerelease area many months ago and didn't get any info (only an answer from Harbs)
Dear Adobe engineers, are there any plans on checkmarks? I think that's an essential thing that can help to produce intiutive-working panels.
Sorry guys. Checkmarks in flyout menus are not supported.
I'll make sure it's on the feature request list.