I believe the id for the Contacts sidebar is #abContactsPanel. You might find some guidance here:

https://searchfox.org/comm-esr115/source/mail/themes/shared/mail/abContactsPanel.css

but I advise starting with css that omits the composition window, then adding a bit at a time and testing each one.

Note that the spacing between contacts in the panel is controlled by the View/Density setting in the main window.

Thank you for your reply. I did have #abContactsPanel in some of my css, but it always seemed to be acting on the headings and not the place where the contacts are.

Anyway, after a very looong session removing and replacing, with no success, I did a Safe Mode open and that worked! So I tested my addons (only have a few) and they didn't affect it, so I was back to the css code again. I removed everything related to the composition window (as you suggested) which was essentially everything below the basic first few commands. That worked. I gradually add them back one at a time, testing each, until I reached this:

/* 6. Contacts - Side panel */

 #abContactsPanel,
 #contactsSidebar .sidebar-header {
  color:            black !important;
  height:		12px !important;
  padding-block:	1px !important; 

}

This was part of a package of commands to change the appearance of the composition window, published in Elevenforum: Thunderbird 115: CSS Customization. I am being specific for the benefit of anyone with the same problem. I went back to that forum and realised that these commands were designed to change the colours, and that I had used it and altered it, thinking I could get the right effect (condensing the Contacts list) by fiddling with it. No luck with that, and I tested it again today by changing the values, but no difference.

The final result is: that code is faulty (because of my alterations) and causes the contacts panel to shrink to one contact only, and does nothing to fix the spacing of the Contacts list, which I now know is unfixable.

Thank you for your guidance. At least I have the whole panel back now, even if it is too spaced with silly little faces wasting space.

The first attached picture shows the sidebar spacing with compact Density and font size 12 px, the default. The second one is with relaxed Density and 12 px. I don't know the code to change the density for just the sidebar, but the compact setting is adequate.

Thanks again for the info about the compact setting. I have already been using that combined with the Layout number in the Config editor, but that is a global setting. The dislike I have is specifically for the Contacts sidebar when composing an email. The names are excessively spaced, and have little head icons beside that I suspect prevent condensing the list at all, and serve no purpose at all. I prefer an unadorned list.

To be honest, I find the composing window and Address Books the most frustrating features of Thunderbird since updating from v102. In particular, they are less customisable than the reading window but there are many aspects I would change if I could find a way of doing so. I am thankful at least for being able to revert to the horizontal view.