[2.x] fix: TextEditor.onbuild race against async Mithril redraw

`oncreate` called `_load().then(() => setTimeout(this.onbuild, 50))`.
The 50ms timer is a guess at how long it takes Mithril's async
`m.redraw()` to flush — under slow first-load conditions (cold CDN,
heavy DOM, additional `_loaders`) the redraw can land *after* the
timer fires, leaving `this.$('.TextEditor-editorContainer')[0]` as
`undefined` when `onbuild` runs. That `undefined` then propagates
into the editor driver (notably breaking Tiptap-based drivers like
fof/rich-text, where it produces a non-functional toolbar with no
editor underneath).

Use `m.redraw.sync()` in `_load` so the DOM is updated before the
promise resolves, and drop the 50ms timer entirely. The `.then` runs
on a microtask outside any Mithril event handler, so `redraw.sync()`
is safe here.

Closes #4612

Author: imorland

framework/core/js/src/common/components/TextEditor.js

@@ -72,9 +72,7 @@ export default class TextEditor extends Component {
   oncreate(vnode) {
     super.oncreate(vnode);
 
-    this._load().then(() => {
-      setTimeout(this.onbuild.bind(this), 50);
-    });
+    this._load().then(this.onbuild.bind(this));
   }
 
   onbuild() {
@@ -96,7 +94,12 @@ export default class TextEditor extends Component {
   _load() {
     return Promise.all(this._loaders.map((loader) => loader())).then(() => {
       this.loading = false;
-      m.redraw();
+      // Synchronous redraw so the editor container is in the DOM before
+      // `onbuild` runs. Mithril's regular `m.redraw()` queues for the next
+      // frame, which previously raced against a 50ms setTimeout and could
+      // leave `this.$('.TextEditor-editorContainer')[0]` undefined on slow
+      // first loads. See flarum/framework#4612.
+      m.redraw.sync();
     });
   }