Display problem for "scale" menu (menu to switch from px to in, mm, ..)

GIMP version: 2.99

Operating System: Ubuntu 18.04.1

Description of the bug

When clicking on the scale on bottom left, the menu doesn't show up well and we have to scroll down to display the different possibilities (px,in, mm,..). The problem only occur on fullscreen mode.

Reproduction

Is the bug reproducible? Always

Reproduction steps:

1. Open a new image or project.
2. Click on the "px" button on bottom left to switch scale.

Expected result: We would want a better display mode like the "zoom" menu on the right of the scale menu.