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