UAC asks for credentials in a "Secure Desktop" mode, where the entire screen is faded out and temporarily disabled, to present only the elevation UI. This is to prevent spoofing of the UI or the mouse by the application requesting elevation. If the application requesting elevation does not have focus before the switch to "Secure Desktop" occurs, then its taskbar icon blinks, and when focussed, the elevation UI is presented (however, it is not possible to prevent a malicious application from silently obtaining the focus).