The ornamental design for a displayscreen or portion thereof with a transitionalgraphicaluserinterface, as shown and described.
7. Display screen or portion thereof with a transitional graphical user interface
7.1: Front, first image in a sequence
7.2: Front, second image in a sequence
7.3: Front, third image in a sequence
The outer perimeters shown in broken lines in design 7 illustrate a display screen or portion thereof and form no part of the claimed design; the remaining broken lines in the figures illustrate portions of the graphical user interface that form no part of the claimed design; the appearance of the transitional image sequentially transitions between the images shown in reproductions 7.1through 7.3; the process or period in which an image transitions to another image forms no part of the claimed design.