Upstream sync

app/assets/javascripts/trix.js

@@ -1,4 +1,30 @@
 // Do not allow file attachments in rich text content
 addEventListener("trix-file-accept", function(event) {
   event.preventDefault();
-})
+});
+
+Trix.config.textAttributes.underline = {
+    style: { "textDecoration": "underline" },
+    inheritable: true,
+    parser: function (element) {
+        var style = window.getComputedStyle(element);
+        return style.textDecoration === "underline";
+    }
+}
+
+document.addEventListener('trix-initialize', function (e) {
+    const trix = e.target;
+    const toolBar = trix.toolbarElement;
+
+    // // Creation of the button
+    const button = document.createElement("button");
+    button.setAttribute("type", "button");
+    button.setAttribute("class", "trix-button trix-button--icon trix-button--icon-underline");
+    button.setAttribute("data-trix-attribute", "underline");
+    button.setAttribute("title", "underline");
+    button.setAttribute("tabindex", "-1");
+    button.innerText = "U";
+
+    // Attachment of the button to the toolBar
+    toolBar.querySelector('.trix-button-group--text-tools').appendChild(button);
+});