preview's sidebar won't go away since I upgraded to 12.6.8

This is utterly broken in Sonoma. If it's supposed to be per tab it's not working and in a way that the least amount of QA would have detected in five minutes.

Example scenario:

1. Open two pdfs in two tabs in the same window
2. Open the sidebar for the first one
3. Switch to the second one
4. Switch back to the first one
5. Close the sidebar
6. Again to the second one
7. It has a sidebar even if Hide sidebar is checked
8. To remove the sidebar you have to open thumbnails or another panel and hide it again

Now think of the same scenario with N tabs, you have to do that N-1 times in order to get rid of the sidebar you opened once.

The only way to avoid this is to remember to close the sidebar before switching to another tab.

Maybe the next release will fix this with AI. One can only hope.

The Preview Hide Sidebar or Show Thumbnails setting is a per-PDF setting and not a global setting for all PDFs, or those that are already open and not frontmost.

Tested: Ventura 13.5.2 and Monterey 12.6.9.

The specific PDF will open in whatever state (Hide Sidebar, Thumbnails) that you last set before closing it and quitting Preview. As I said, it is a per-PDF setting, and Hiding the Sidebar on one PDF does not mean that any other PDF last saved with Thumbnails will then open with no Sidebar.

When you have two PDFs open, and their windows merged as tabs, Preview reserves a blank Sidebar area for the PDF whose Sidebar is shown hidden, and the other PDF tab has Thumbnails chosen. This gives the illusion that the Hide Sidebar is not working, but it is for the current document you have shown.

There is no assurance with future releases of macOS that Apple will continue to provide Preview as it has always worked in earlier releases of macOS. It is their operating system and they can, have, and will change things.