We claim the ornamental design for a display screen or portion thereof with an animated graphical user interface, as shown and described.
FIG. 1 is a front view of a first image in a sequence for a display screen or portion thereof with an animated graphical user interface showing our new design; and,
FIG. 2 is a front view of a second image thereof.
The outermost broken line rectangle shows the display screen or portion thereof. All other broken lines show portions of the animated graphical user interface and form no part of the claimed design.
The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1-2. The process or period in which one image transitions to another forms no part of the claimed design.