IIUC that is the expected behaviour. The sidebar has its own scroll bar, so that the controls inside of it remain kind of "sticky"

Change #1234375 had a related patch set uploaded (by Rehan_khan_78; author: Rehan_khan_78):

[mediawiki/skins/Vector@master] Fix: Add scroll support to pinned appearance menu

https://gerrit.wikimedia.org/r/1234375

I vaguely remember someone saying they wanted it as the first item in the menu. I'd suggest chatting through and agreeing on expected behaviour before going into solution mode. Since this is not a permanent feature I'd recommend avoiding long term solutions to the menu itself here. It was only ever designed with three options in mind!

"Hi @Jdlrobson-WMF, thanks for the feedback. I see your point about avoiding long-term changes to the core menu height for a temporary feature. Since the issue is primarily visible in the pinned sidebar state where the default dropdown scroll logic might not apply, what behavior would the team prefer?

Options:

Reordering: Move the Birthday toggle to the top of the menu so it’s always visible.

Temporary CSS: Add a scoped overflow rule that only exists while the WP25 feature is active.

UI Content: Shorten the help text for Dark/Wide modes to save vertical space.

Looking forward to the team's guidance!"

@Rehan_khan_78: That looks a bit like AI output copied into the previous comment?

"Hi @Aklapper, I did use an AI tool to help format my response and improve the clarity of my English, as I wanted to make sure my suggestions for the team were professional and easy to understand. However, the options I proposed (reordering the menu, scoped CSS, or shortening text) are based on my own analysis of the bug and the feedback I received from Jon in Gerrit. I'm fully committed to the task and want to make sure the solution aligns with the team's preferences!"

"Hi @Jdlrobson-WMF , T415809 seems closely tied to the layout bug in T415709. Since I'm already working in the Vector code, I'm happy to tackle this feedback issue as well. Should I take it on, or do you have a specific fix in mind?"

In T415709#11565677, @Rehan_khan_78 wrote:

"Hi @Jdlrobson-WMF , T415809 seems closely tied to the layout bug in T415709. Since I'm already working in the Vector code, I'm happy to tackle this feedback issue as well. Should I take it on, or do you have a specific fix in mind?"

We are still discussing these and haven't decided if these are bugs we need to fix. We are working to a very tight timeline so trying to prioritize only the essential work (bugs marked as "High" and "Medium"), right now. Thanks in advance for your patience