Give any email a title that makes sense.

{
  "manifest_version": 3,
  "name": "Retitle for Gmail",
  "version": "1.1.0",
  "description": "Change the title (subject line) of any email in Gmail. Your titles sync to every computer where you are signed in to Chrome.",
  "permissions": ["storage"],
  "host_permissions": ["https://mail.google.com/*"],
  "content_scripts": [
    {
      "matches": ["https://mail.google.com/*"],
      "js": ["content.js"],
      "css": ["content.css"],
      "run_at": "document_idle"
    }
  ]
}

// Retitle for Gmail - content script (v1.1, synced).
// Each rename is its own chrome.storage.sync item: "rt:<original subject>" -> "your title".
// Chrome Sync copies these to every browser signed in to the same Google account.
(function () {
  var PREFIX = "rt:";
  var OLD_KEY = "retitle:map"; // v1.0 stored renames on this computer only
  var map = {};
  var queued = false;

  function failed() {
    if (chrome.runtime.lastError) {
      window.alert("Retitle could not save: " + chrome.runtime.lastError.message);
      return true;
    }
    return false;
  }

  // Show your title in place of the original, and remember the original.
  function apply(el) {
    var text = el.textContent;
    var shown = el.dataset.rtShown;
    var orig = shown !== undefined && text === shown ? el.dataset.rtOrig : text;
    var want = map[orig] || orig;
    if (text !== want) el.textContent = want;
    el.dataset.rtOrig = orig;
    el.dataset.rtShown = want;
  }

  function rename(h2) {
    var orig = h2.dataset.rtOrig || h2.textContent;
    var current = map[orig] || orig;
    var next = window.prompt(
      "New title for this email (it follows you to every computer signed in to Chrome).\nLeave empty to restore the original.",
      current
    );
    if (next === null) return;
    next = next.trim();
    if (!next || next === orig) {
      delete map[orig];
      chrome.storage.sync.remove(PREFIX + orig, failed);
    } else {
      map[orig] = next;
      var item = {};
      item[PREFIX + orig] = next;
      chrome.storage.sync.set(item, failed);
    }
    scan();
  }

  function addButton(h2) {
    var host = h2.parentElement;
    if (!host || host.querySelector(".rt-btn")) return;
    var b = document.createElement("button");
    b.type = "button";
    b.className = "rt-btn";
    b.textContent = "Rename";
    b.title = "Change this email's title";
    b.addEventListener("click", function (e) {
      e.preventDefault();
      e.stopPropagation();
      rename(h2);
    });
    h2.insertAdjacentElement("afterend", b);
  }

  function scan() {
    queued = false;
    // Subject line inside an open conversation.
    document.querySelectorAll("h2.hP").forEach(function (h2) {
      apply(h2);
      addButton(h2);
    });
    // Subject line in inbox and list rows.
    document.querySelectorAll("tr.zA span.bog").forEach(apply);
  }

  function queue() {
    if (queued) return;
    queued = true;
    requestAnimationFrame(scan);
  }

  // A rename made on another computer arrives here a few seconds after it is saved.
  chrome.storage.onChanged.addListener(function (changes, area) {
    if (area !== "sync") return;
    for (var k in changes) {
      if (k.indexOf(PREFIX) !== 0) continue;
      var orig = k.slice(PREFIX.length);
      if (changes[k].newValue) map[orig] = changes[k].newValue;
      else delete map[orig];
    }
    scan();
  });

  function load() {
    chrome.storage.sync.get(null, function (all) {
      map = {};
      for (var k in all) {
        if (k.indexOf(PREFIX) === 0) map[k.slice(PREFIX.length)] = all[k];
      }
      scan();
      new MutationObserver(queue).observe(document.body, {
        childList: true,
        subtree: true,
        characterData: true
      });
    });
  }

  // One-time move of v1.0 renames from this computer into synced storage.
  chrome.storage.local.get(OLD_KEY, function (r) {
    var old = r[OLD_KEY];
    if (!old) return load();
    var items = {};
    for (var k in old) items[PREFIX + k] = old[k];
    chrome.storage.sync.set(items, function () {
      if (!chrome.runtime.lastError) chrome.storage.local.remove(OLD_KEY);
      load();
    });
  });
})();

.rt-btn {
  margin-left: 10px;
  padding: 2px 10px;
  border: 1px solid #c4c7c5;
  border-radius: 999px;
  background: #fff;
  color: #1a237e;
  font: 500 12px/20px "Google Sans", Roboto, Arial, sans-serif;
  cursor: pointer;
  vertical-align: middle;
}
.rt-btn:hover { background: #e8eaf6; }
.rt-btn:focus-visible { outline: 2px solid #3f51b5; outline-offset: 2px; }