UX: Improve loading-slider behavior (#29995)

- Use `requestAnimationFrame` when transitioning from `ready` -> `loading`. The previous `next()` implementation was unreliable, particularly in Safari, and would cause the loading slider to jump backwards instead of forwards

- Double the minimum transition time to 200ms. This avoids the rolling average being skewed too much by routes which load quickly without network access.

app/assets/javascripts/discourse/app/components/page-loading-slider.gjs

@@ -2,7 +2,7 @@ import Component from "@glimmer/component";
 import { tracked } from "@glimmer/tracking";
 import { on } from "@ember/modifier";
 import { action } from "@ember/object";
-import { cancel, next } from "@ember/runloop";
+import { run } from "@ember/runloop";
 import { service } from "@ember/service";
 import { htmlSafe } from "@ember/template";
 import { eq } from "truth-helpers";
@@ -23,12 +23,16 @@ export default class PageLoadingSlider extends Component {
   willDestroy() {
     super.willDestroy(...arguments);
     this.loadingSlider.off("stateChange", this, "stateChange");
+    if (this._deferredStateChange) {
+      cancelAnimationFrame(this._deferredStateChange);
+      this._deferredStateChange = null;
+    }
   }
 
   @bind
   stateChanged(loading) {
     if (this._deferredStateChange) {
-      cancel(this._deferredStateChange);
+      cancelAnimationFrame(this._deferredStateChange);
       this._deferredStateChange = null;
     }
 
@@ -36,7 +40,9 @@ export default class PageLoadingSlider extends Component {
       this.state = "loading";
     } else if (loading) {
       this.state = "ready";
-      this._deferredStateChange = next(() => (this.state = "loading"));
+      this._deferredStateChange = requestAnimationFrame(() => {
+        run(() => (this.state = "loading"));
+      });
     } else {
       this.state = "done";
     }

app/assets/javascripts/discourse/app/services/loading-slider.js

@@ -7,7 +7,7 @@ import { bind } from "discourse-common/utils/decorators";
 
 const STORE_LOADING_TIMES = 5;
 const DEFAULT_LOADING_TIME = 0.3;
-const MIN_LOADING_TIME = 0.1;
+const MIN_LOADING_TIME = 0.2;
 
 const STILL_LOADING_DURATION = 2;