I claim the ornamental design for a display screen or portion thereof with transitional graphical user interface, as shown and described.
FIG. 1 is the first image in a sequence for a display screen or portion thereof with transitional graphical user interface showing our new design;
FIG. 2 is the second image thereof;
FIG. 3 is the third image thereof;
FIG. 4 is the fourth image thereof; and,
FIG. 5 is the fifth image thereof.
The outermost broken lines in the figures depict the display screen or portion thereof and form no part of the claimed design.
The remaining broken lines in the figures depict portions of the transitional graphical user interface which form no part of the claimed design.
The appearance of the transitional graphical user interface sequentially transitions between the images shown in FIGS. 1 through 5. The process or period in which one image transitions to another forms no part of the claimed design.