The ornamental design for a displayscreen or portion thereof with transitional graphical user interface, as shown and described.
FIG. 1 is a front view of the first state in a sequence for a display screen or portion thereof with transitional graphical user interface showing our new design;
FIG. 2 is a front view of a second state thereof;
FIG. 3 is a front view of a third state thereof;
FIG. 4 is a front view of a fourth state thereof;
FIG. 5 is a front view of a fifth state thereof; and,
FIG. 6A-FIG. 6B is a front view of a sixth state thereof.
The outermost broken line rectangle illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines, including all text, illustrate portions of the graphical user interface that form no part of the claimed design. The appearance of the transitional graphical user interface sequentially transitions between the images shown in FIGS. 1-6. The process or period in which one image transitions to another forms no part of the claimed design.