To reproduce, run the following on Linux:

1) Launch SceneBuilder
2) Expand the Controls section of the Library
3) Drag a Button from the Controls section of the Library into the Pane in the main window
4) Click on the arrow of the "Font" combo box
5) Click on the arrow for the Style combo box and then try to move the cursor down into the popup to select a style

The popup will dismiss before the cursor is able to enter it. Sometimes if you drag the mouse down while selecting it such that you are within the bounds of the popup before you release it, it will work and allow you to select, but it is difficult to do.

Note that the Size combo box has the same problem.

This problem does not happen on Mac or Windows. It is specific to Linux.