Steps to reproduce:

1) Add a RadForm with 3 buttons. Apply Fluent Theme to all.

2) In the click handler of the first two buttons, set enable to false for the button that was clicked. Third button, do nothing.

3) Run app. Click all 3 buttons.

Desired Result:

Focus indicator should be hidden for previous-focused but now-disabled controls.

Actual Result:

Blue Fluent focus-indicator border is still showing (my user's are confused and complaining), see attachment.

[May happen on other themes too, didn't check.]

Had to implement a work-around...

Work around: 

Set border element to off for every programmatically disabled control: this.radButton1.ButtonElement.BorderElement.Visibility = ElementVisibility.Hidden;