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 first image of a first embodiment 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 image of the first embodiment thereof;
FIG. 3 is a front view of a third image of the first embodiment thereof;
FIG. 4 is a front view of a first image of a second embodiment thereof;
FIG. 5 is a front view of a second image of the second embodiment thereof; and,
FIG. 6 is a front view of a third image of the second embodiment thereof.
The broken line showing of a display screen or portion thereof with transitional graphical user interface is included for the purpose of illustrating portions of the article and forms no part of the claimed design.