In every other tab, the scroll bar on the side of the window is a custom gray color. But in the Realms tab for both Bedrock and Java, the scroll wheel uses, in this case, the macOS default scroll wheel. This does not affect the function of the launcher, it is just a visual bug.
I cannot reproduce in macOS 13 – this could be an issue only present in older versions of macOS (doesn't mean it shouldn't be fixed!).