DEV: Remove `after-transition` function (#21142)

All supported browsers use `transitionend` event now, so this code is not necessary and makes it difficult to use that event in tests (you'd have to trigger all variants to cover the bases)

That function was used only in core (no hits in all-the*) in two places, so I think it's rather safe to just trash it without deprecating it first.

(History Corner – this helper was originally added in the initial commit of Discourse! 1839614bcc/app/assets/javascripts/discourse/components/transition_helper.js.coffee)

app/assets/javascripts/discourse/app/components/composer-body.js

@@ -7,7 +7,6 @@ import discourseComputed, {
 import Component from "@ember/component";
 import Composer from "discourse/models/composer";
 import KeyEnterEscape from "discourse/mixins/key-enter-escape";
-import afterTransition from "discourse/lib/after-transition";
 import discourseDebounce from "discourse-common/lib/debounce";
 import { headerOffset } from "discourse/lib/offset-calculator";
 import positioningWorkaround from "discourse/lib/safari-hacks";
@@ -181,7 +180,7 @@ export default Component.extend(KeyEnterEscape, {
     };
     triggerOpen();
 
-    afterTransition($(this.element), () => {
+    this.element.addEventListener("transitionend", () => {
       triggerOpen();
     });
 

app/assets/javascripts/discourse/app/components/quote-button.js

@@ -1,4 +1,3 @@
-import afterTransition from "discourse/lib/after-transition";
 import { propertyEqual } from "discourse/lib/computed";
 import { ajax } from "discourse/lib/ajax";
 import { popupAjaxError } from "discourse/lib/ajax-error";
@@ -412,7 +411,9 @@ export default Component.extend(KeyEnterEscape, {
 
           this.editPost(postModel);
 
-          afterTransition(document.querySelector("#reply-control"), () => {
+          document
+            .querySelector("#reply-control")
+            ?.addEventListener("transitionend", () => {
               const textarea = document.querySelector(".d-editor-input");
               if (!textarea || this.isDestroyed || this.isDestroying) {
                 return;

app/assets/javascripts/discourse/app/lib/after-transition.js

@@ -1,34 +0,0 @@
-/**
-  CSS transitions are a PITA, often we need to queue some js after a transition, this helper ensures
-  it happens after the transition.
-
-  SO: http://stackoverflow.com/questions/9943435/css3-animation-end-techniques
-**/
-let dummy = document.createElement("div"),
-  eventNameHash = {
-    webkit: "webkitTransitionEnd",
-    Moz: "transitionend",
-    O: "oTransitionEnd",
-    ms: "MSTransitionEnd",
-  };
-
-let transitionEnd = (function () {
-  let retValue;
-  retValue = "transitionend";
-  Object.keys(eventNameHash).some(function (vendor) {
-    if (vendor + "TransitionProperty" in dummy.style) {
-      retValue = eventNameHash[vendor];
-      return true;
-    }
-  });
-  return retValue;
-})();
-
-export default function (element, callback) {
-  return $(element).on(transitionEnd, (event) => {
-    if (event.target !== event.currentTarget) {
-      return;
-    }
-    return callback(event);
-  });
-}