Mac Preview not collapsing pages since I upgraded to Tahoe 26.1

Echoing the question, but I suspect you mean the little caret at the right edge of the sidebar next to the filename. I don't think I've ever paid attention to that. I have two Macs side by side, one on 15.7.1 and the other on 26.1.

It's there in Thumbnail view when you hover over the spot in Sequoia, and clicking it collapses the document to a single page:

However, the filename itself is not there in Tahoe, and thus there's no caret to click.

I notice that if you set the View menu to Table of Contents, the filename and caret are there, and clicking it collapses the ToC.

However, if when collapsed you set the View back to Thumbnail, the filename disappears and all the page thumbnails are there.

Seems like a Tahoe bug. Bugs can be reported to Apple here:

Feedback - macOS - Apple

Kris_Naptown wrote:

I regularly collapse my documents in Preview because it's the easiest way for me to add them to other docs. I do this all day long for work, so it really comes in handy, but now the only options I am getting in the dropdown menu are Hide Sidebar, Thumbnails, Table of Contents, etc. The down arrow next to the dropdown list just gives me the same options. Hopefully it will be resolved but in the meantime anyone else have this issue and were you able to fix it?

re: Preview

what does that mean..."collapse my documents" ?

If this is in reference to the yellow minimize button— I have never used this feature in any macOS. There are better ways for a more effective work flow.

If something changed—

To be proactive you can file a bug report / submit your Apple Feedback here: Product Feedback - Apple

I typically have my preview>settings as follows—

I see no difference in Preview.app macOS 26.1

four finger swipe down (ref trackpad setting) for exposé....is often over looked for easy access in most any application.