Currently in `switchSidebarView` there is code that attempts to handle the `outline` or `attachments` being disabled. However, given the placement of it, that code does not actually accomplish anything. Even more strange is the fact that the way that the buttons are disabled, that code won't ever be hit.
(Looking back with `git blame`, it seems that it has never worked as you'd expect.)
Hence this patch, which correctly disables switching to the `outline` or `attachments` views when they are disabled.
Once PR 6314 restores the 'pagemode' hash parameter, this patch thus makes it impossible to switch to a disabled sidebar view by modifying the 'pagemode' hash of an already loaded document.
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