A tiny semi-transparent outline around the tooltip, made visible by hovering on current repo dropdown button

### The problem

When you hover on top of the current repository drop down button, there's a tiny outline around the tooltip, which has a padding of about 5 px outside the tooltip. I saw this by accident.

### Release version

Version 3.4.16 (x64) (I have seen this in previous versions as well)

### Operating system

Windows 11 Home Single Language

### Steps to reproduce the behavior

1. Open Github Desktop
2. Use mouse and hover on current repo dropdown button
3. If you watch closely, there's an outline around the tooltip

### Log files

_No response_

### Screenshots

This is the button that i am talking about


This is the tooltip that appears after hovering on top of the button


If you look closely, there's an outline around the tooltip as shown below (just inside of the red dots, the outline is partially transparent)


### Additional context

_No response_