Both the `Guest` and `HTMLIntegration` maintained separate state for whether
side-by-side mode was active or not. These states could get out of sync if the
side-by-side feature flag was disabled when the guest first called
`fitSideBySide` and then later enabled, as a result of the user logging into a
different profile. In that case side-by-side would be enabled from the
integration's perspective, but `Guest._sideBySideActive` would remain false.

More generally this mismatch could occur in future if, for any reason, the
integration decided to disable side-by-side outside of the context of a call to
`fitSideBySide`.

The result of this error was that clicking somewhere in the page would cause the
sidebar to close, because the Guest thought that side-by-side was disabled, even
though it was actually active.