sidebar: Ensure plus icon button stays visible when menu is open (#58482)

Similar treatment to the ellipsis menu, where we keep the icon button
that triggers the menu visible, so we know where the menu is anchored
to. This was missing from a recent PR where we improved the drafts UX.

Release Notes:

- N/A
f70edafd2fDanilo Leal committed on 6/3/2026, 11:23:01 PM· committed by GitHubparent971977a
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