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 a display screen or portion thereof with transitional graphical user interface showing a first image in a sequence according to a first embodiment of the claimed design;
FIG. 2 is a front view of a second image in the sequence thereof;
FIG. 3 is a front view of a display screen or portion thereof with transitional graphical user interface showing a first image in a sequence according to a second embodiment of the claimed design; and,
FIG. 4 is a front view of a second image in the sequence thereof.
The appearance of the transitional image sequentially transitions between the images shown in FIG. 1 to FIG. 2 in the first embodiment, and from FIG. 3 to FIG. 4 in the second embodiment. The process or period in which an image transitions to another image forms no part of the claimed design.