Most UI toolkits I've used have a notion of an "indefinite" progress bar. In the past these have sometimes done like a horizontal barber pole animation. In fact

Most UI toolkits I've used have a notion of an "indefinite" progress bar. In the past these have sometimes done like a horizontal barber pole animation. In fact, I have memories of the one in Thunderbird doing this; it would be a normal progress bar while the email's data was sent, and transition to an indefinite bar while it awaited the server's response. After all … there's no "max" or "value" to the progress … we're just waiting for a singular response. Windows, in the glassy Vista era, had a green one that would shimmer & slide a small faded green blob (for lack of a better word) across the bar. (IMO better than what Explorer did for years, which was to show a definite progress bar, i.e., one with a shaded portion indicating some % completion … and then just lie about it. It would just fill the bar up with an exponential decay towards 100%, but never actually get there … and was really just an animation.)