An eyedropper next to the Cryptomatte effect's Selection arms the viewer pick, and the status
line says how to use it. It stays armed until Esc or the button (Esc in a text field or a menu
only ends that); disarming, New, Open and Revert end it and let go of the session's probe.
The app runs the pick's bookkeeping at the start and end of each pass, the viewer hands the
pointer to the pick while it is armed and opens its context menu for a right-click the pick
forwarded, and a Selection cut off by a narrow Effect Controls panel shows whole on hover.
UI tests: ui_crypto_pick (arming, hover, clicks, modifiers, drags, waiting clicks, status,
highlight) and ui_crypto_pick_undo (one undo step per pick, the probe kept over undo).