Violentmonkey script for Gmail email lists, see: https://lucascosti.com/blog/2019/04/making-split-pane-mode-usable-in-the-new-gmail-interface/

Gmail-List-userscript.js

// ==UserScript==
// @name Work Gmail multi-line email list
// @description Makes sure that the new (late 2018) Gmail interface always shows the three-line previews of email lists (mainly for split-pane mode).
//  The new Gmail behaviour changes this to one-line when the list pane is wide enough (but not wide enough for my liking).
// @author Lucas Costi
// @namespace https://lucascosti.com
//// The scope below only matches the second Gmail account (which is always my work one).
// @match https://mail.google.com/mail/u/1/*
// @grant none
// ==/UserScript==
//
//

// only run once for each page load:
if (window.top != window.self) //don't run on frames or iframes
{
  //Optional: GM_log ('In frame');
  return;
}

// Turn on/off console log debugging
DEBUG = false;

DEBUG && console.log("➡️ Starting to check for the div... ");
// start the check function and loop it every 200ms until it is cleared.
var myAwesomeGmailEmailListCheckScript = setInterval(fixEmailListDiv, 200);

// Check the div for the class we want, and add it if it isn't present. 
function fixEmailListDiv() {
  // Find the div that specifically has a class added/removed to control the Gmail behaviour.
  // It doesn't have an ID or unique class. The only thing unique about it is the below Gmail jasaction attribute.
  // The Gmail div jsaction attribute has changed over time.
  // before 2020-09-27: [jsaction="Ic0gz:.CLIENT;oSin0b:.CLIENT"]
  // after  2020-09-27: [jsaction="oehdpb:.CLIENT;UXdbee:.CLIENT"]
  var emailListDiv = document.querySelector('[jsaction="oehdpb:.CLIENT;UXdbee:.CLIENT"]');
  // If the div doesn't exist yet, we'll loop again.
  if (emailListDiv == null) {
    DEBUG && console.log("div not found... 🔎");
  } else {
    DEBUG && console.log("div FOUND!! 👏");
    if (!emailListDiv.classList.contains("Zs")) {
      DEBUG && console.log("Class not present on div. Adding now... 🚀");
      emailListDiv.classList.add("Zs");
      // Setup an observer if it doesn't exist
      if (!observer) {
        DEBUG && console.log("Starting observer...");
        // Set up a mutation observer on the div to intercept any changes made to remove the class.
        var observer = new MutationObserver(fixEmailListDiv);
        // Options for the observer (which mutations to observe).
        var observerOptions = {
          attributes: true
        }
        // Start observing the div for mutations.
        observer.observe(emailListDiv, observerOptions);
      }
    } else {
      DEBUG && console.log("Class present on div. ✅");
    }
    // Clear the loop interval, because we have processed at least 1 div.
    DEBUG && console.log("Clearing loop. 🏁");
    clearInterval(myAwesomeGmailEmailListCheckScript);
  }
}