The problem is that when Show separate sidebar modes for forum and chat is set to “always” and you are using the chat drawer, there’s no way to access the all-channel search, because we don’t add any chat links (search or otherwise) to the sidebar in this case and there’s no all-channel search in drawer.
I think this makes sense, it’ll close this gap and make channel-search easier to access for folks with crowded sidebars (even if they do have chat links there).