Can't scroll through the units present in a city when spying

[hugomflavio]

Describe the bug Can't scroll through the units present in a city when spying. Only the first row is visible.

To Reproduce Steps to reproduce the behavior:

1. Spy a city with ~ 20 units inside
2. Try to scroll down on the units list
3. Profit
4. See error

Expected behavior The units list should scroll down.

Screenshots (the scroll bar is there, but neither scrolling nor clicking and dragging worked)

Platform and version (please complete the following information):

• OS: Linux
• Freeciv21 version: 3.1-beta.1
• Ruleset/Longturn game (if applicable): LT83

[lmoureaux]

Related to #2286