I’m not sure if this is the intended behavior, but if the Sidebar is hidden and you drag the edges of a window to resize it, the closed Sidebar automatically reappears (both in Font View and Edit View).
I’m on a M4 MBA with macOS 26.1, Glyphs 3.5 (3507) — with an external screen. This may be particularly important because when I close and reopen the laptop lid (switching from clamshell mode to dual-screen mode), I find that all hidden Sidebars on the external display have opened back.
As for my use case, it is distracting while trying to manually organize multiple windows on a single screen.