The ornamental design for a displayscreen or portion thereof with transitional graphical user interface, as shown and described.
FIG. 1 is a first image in a sequence of a display screen or portion thereof with transitional graphical user interface showing our new design;
FIG. 2 is a second image thereof;
FIG. 3 is a third image thereof;
FIG. 4 is a fourth image thereof;
FIG. 5 is a fifth image thereof; and,
FIG. 6 is a sixth image thereof.
The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1-6. This process or period forms no part of the claimed design. The long and short dashed broken line showing of a mobile terminal in FIGS. 1-6 forms no part of the claimed design. The mobile terminal, display screen, and the text, numbers, symbols and images of the graphical user interface shown in broken line in FIGS. 1-6 depict subject matter that forms no part of the claimed design.