Press F8. That key toggles the state.

Matt, that is a supremely useful suggestion for when I want to eliminate obvious or known spam from the inbox message list display. However, my problem involves a feature I mis-named as the "preview" pane-- a pop-up window that appears whenever I simply click on a single message, as in the attached screen-shot'.

So, I searched again for something in the documentation, a feature I could turn off and on, as with your F8 control. Sorry to have been so imprecise about the appropriate term "pop-up" (instead of "preview panel"), and look forward to your comment.

What you want to change is the setting for how messages are displayed. You have it set to show messages in new windows. Apparently, you want them to be shown in the message pane instead.

≡ button > Settings > General > Reading & Display > Open messages in ... an existing message window

That image is of a message in it's own window, but I am left confused as that is an action that should only ever occur post a double click.

To follow Ricks advice,

You will get a change in action on double clicking but not single clicking as you specified.

SO I am left to ponder the issue of a single click. Windows has a setting for this Could it be you have enabled this

There is also apparently an issue with some mice on Windows. I saw HP and Logitech mentioned specifically) that make single clicks into double clicks. (I had never heard of it until I tried Google with the issue tonight) It has become such an issue someone has developed this software to modify how widows manages the mouse clicks. https://apps.microsoft.com/detail/9pdgm7nl2ff2?hl=en-us&gl=CH&ocid=pdpshare

It is apparently free software and the source is available here https://github.com/nenning/DoubleClickFix as well as some basic instructions for use (scroll down past the source file lists)

Good catch, Matt.

Matt and Rick-- thank you!

RE: Click and Double=Click My Logitech mouse driver is a bit flaky, since a single click sometimes yields a double response, as though I had clicked twice.

RE: Pop-up windows. Will check my TB setup for opening each message in a separate window, although I do not recall ever having chosen "open in a separate window".

RE: Why pop-up windows are more than annoying Viewing anything by opening the message is dangerous, considering the more recent attack modes.

RE: Click and Double=Click My Logitech mouse driver is a bit flaky, since a single click sometimes yields a double response, as though I had clicked twice.

Some mouse models have a (middle) scroll button which can also be used for a double-click by pressing it. Maybe you did this inadvertently?

I may understand one of your objectives better now: you don't want any preview because, for privacy or security reasons, you don't want any message to open automatically. A message will open whether you click or double-click on it in the message table and whether you have preferences set to show it in the message pane, a new tab, or a new window. The only way not to open a message is not to click on it in the message table.

If you are not already familiar with the setting not to load remote images, you may want to consider using it.