Apple Mail thread disclosure triangle?

I can confirm that, for me, toggling the "Organize by Conversation" setting under the View Menu off and then back on reset Apple Mail to correctly display the view column with the dual arrows and then display the disclosure triangles beside mail conversations.

Unfortunately the setting fails to hold for me. The disappearance of the Conversation column (the column with the dual arrows) has been random and so to isolate exactly what causes the "Organize by Conversation" column to disappear along with the disclosure triangles has been a challenge. However, it repeatedly occurs and I need to then uncheck the option here:

Mail -> View -> Organize by Conversation

and then re-check it. That then causes the column with the dual arrows to reappear and for conversations to be correctly displayed. Certainly seems like a bug to me.

Early 2015 MacBook Pro

macOS 11.1

Do you have "Organize by Conversation" selected in the View menu? I toggled mine and the column goes away if you don't have "Organized by Conversation" set.

For conversations, there is no triangle to expand and collapse. But fortunately, the right arrow key works to expand, and left arrow to collapse.

I had the same problem. I turned off "Organize by Conversation" and then turned it back on and the column/arrows appeared.

Steven Beaty wrote:

Yes, I see the triangle (well now downward ^) on the mailboxes as before. What I'm not finding it that on a message thread. Here's a thread with no way that I can find to drop down the messages and see them individually:

https://discussions.apple.com/content/attachment/592776cd-d5f7-45e6-a377-8a48e1545aa7

I see the unread circle and a list of respondents, but no way to show the messages individually as was always there before. Thanks!

All I can say by the isolated screen shot you offer up— I have no idea what you are trying to illustrate/ and or the real issue.

Did you read the HT link as posted above: from <Read emails in Mail on Mac>

Change how emails are shown

In the Mail app  on your Mac, do any of the following:

• Turn off conversations to ungroup messages: Choose View > Organize by Conversation. Conversationsare off if there’s no checkmark next to the command.
• Sort messages: Choose View > Sort By, then choose an attribute, such as From or Size, and a sort order.
• Show the message preview on the side or bottom: Choose View > Show Side Preview (a checkmark indicates the preview is on the side). 
• Show more details: Choose View, then choose details to show, such as Date and Time or Message Size (a checkmark indicates a detail is shown).
• Show messages with dark backgrounds (when using Dark Mode on your Mac): Choose Mail > Preferences, click Viewing, then select “Use dark backgrounds for messages.”
• You can switch to a light background while viewing or writing a message. Choose View > Message > Show with Light Background. To see the message with a dark background again, choose View > Message > Show with Dark Background. 
• Resize the message list: Drag the bar that separates the message list and the message preview to the left or right.

Alternatively, you can use column layout to view messages. See Use column layout.

No conversations there. Thanks for everyone's help; it appears I'm asking for a feature that was available but isn't now.

As I stated above, the Conversations column is not manageable. It should be there and you can't remover (or add) it. No idea why it doesn't show for you.

Here is the help page which describes the column layout. Use column layout in Mail on Mac - Apple Support

You can move it around, so maybe look way off the screen to the right.

Steven Beaty wrote:

Anyone know where it went and how to get it back? I used it all the time... Thanks!

Try Control click on the mailbox for some options

Read emails in Mail on Mac

If you move your mouse to the right side to reveal the + you can add/modify

https://support.apple.com/guide/mail/create-or-delete-mailboxes-mlhlp1021/14.0/mac/11.0

Select the conversation and all of the messages should be listed in the viewer pane.

I have the same question. I do not see that green arrow in a circle like you have listed above. I tried checking all of the possible viewing columns but cannot get that. It makes it difficult to work quickly and respond to one specific email in a thread. I also have the unread blue dot but do not have the expand arrow.

leroydouglas commented in part:

"Try Control click on the mailbox for some options"

++++

First, I have been completely unable to see any options following your recommendation. For me I have Mail set to "Use Column Layout" with "Bottom View" so in case that was not allowing me to see what you see I changed the View setting to all the available options for viewing but none have allowed me to see what you see. Do you have any thought as to why I fail to see the same thing?

I am running from a clean install of Big Sur so did not inherit any ugly leftovers from my Mojave setup.

This entire issue has become rather frustrating for me. It was fine to toggle the setting for "Organize by Conversation" once in the hope that would correct the problem but, for me, the column with the dual arrows shown here:

regularly disappears:

which requires the toggling of the "Organize by Conversation" setting to make it reappear.

I have not been able to track down circumstances which cause it to disappear because I am not always looking at the Mail application's windows so can't say what I might have been running or doing when the column disappeared.

Does anyone have suggestions for how I can isolate what is causing the setting to not hold?

It is still there, it just looks a little different: