Bug: Collapsible sidebar not working on theme compatibility mode
The new sidebar collapse feature introduced in the latest update works inside the community, but it isn't available on custom pages using the Fluent Community wrapper template (Full Width or Boxed). It would be great if this functionality were consistent across all pages using the wrapper.
We are working on it. Thanks for reporting.
The issue has already been fixed in dev; the fix will be available in the next release.