💄 Add tooltip element to XUL menugroups

components/menus/content/xul-menu.js

@@ -29,6 +29,11 @@
 		this
 	);
 
+	Services.scriptloader.loadSubScript(
+		"chrome://dot/content/widgets/xul-menugroup.js",
+		this
+	);
+
 	class MozMenu extends MozMenuBaseMixin(
 		MozElements.MozElementMixin(XULMenuElement)
 	) {

components/menus/content/xul-menugroup.js

@@ -3,7 +3,49 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 {
-	class MozMenuGroup extends MozXULElement {}
+	class MozMenuGroup extends MozXULElement {
+		/**
+		 * The menu group's tooltip element
+		 */
+		get groupTooltip() {
+			return /** @type {BrowserTooltip} */ (
+				this.querySelector("tooltip") ||
+					document.createXULElement("tooltip", {
+						is: "browser-tooltip"
+					})
+			);
+		}
+
+		/**
+		 * The current active menuitem
+		 * @type {ReturnType<typeof MozMenuItemBaseMixin<Constructor<XULElement>>>["prototype"]}
+		 */
+		get activeItem() {
+			return this.querySelector("[_moz-menuactive='true']");
+		}
+
+		/**
+		 * Updates the menugroup's tooltip text
+		 */
+		_updateTooltipText() {
+			if (this.groupTooltip.state == "closed") return;
+
+			if (this.activeItem) {
+				this.groupTooltip.label = this.activeItem.label;
+			}
+		}
+
+		connectedCallback() {
+			this.setAttribute("tooltip", "_child");
+
+			this.prepend(this.groupTooltip);
+
+			this.groupTooltip.addEventListener(
+				"popupshowing",
+				this._updateTooltipText.bind(this)
+			);
+		}
+	}
 
 	customElements.define("menugroup", MozMenuGroup);
 }