Bug 1720050 - Ensure to listen to transition events on <select>, not bubbled events from options. r=Gijs

Options can change styles for a variety of reasons and we don't want
transitions on them to re-update the whole menulist.

Differential Revision: https://phabricator.services.mozilla.com/D121353

toolkit/actors/SelectChild.jsm

@@ -342,7 +342,10 @@ SelectContentHelper.prototype = {
         }
         break;
       case "transitionend":
-        if (SUPPORTED_SELECT_PROPERTIES.includes(event.propertyName)) {
+        if (
+          this.element === event.target &&
+          SUPPORTED_SELECT_PROPERTIES.includes(event.propertyName)
+        ) {
           this._updateTimer.arm();
         }
         break;