Manual testing - chartviewer invisible content in ComboBox list

In the manual test chartviewer, all ComboBoxes present an invisible list when left-clicked. When you left-click one of the ComboBoxes, it looks like nothing happens, but if you left-click in the area where a ComboBox option is meant to be, you are actually selecting that option. It is simply invisible.

Steps to reproduce:

1. Left-click the ComboBox under "View" that says "1 chart". Notice that the ComboBox list does not appear.
2. Left-click the text that says "Animations". Notice that the option for 16 charts has been selected.
3. Repeat step 1 and step 2, left-clicking with varying distances from the ComboBox under "View". Notice that the different options are selectable, but invisible.

Expected behavior:

The drop-down list of the ComboBoxes should be visible.

Observed behavior:

The drop-down list of the ComboBoxes are invisible, which makes it impossible to see what options are presented, although selecting the options is still possible by left-clicking the correct area.