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

[imorland]

Summary

TextEditor.oncreate schedules onbuild via a hardcoded setTimeout(..., 50) after _load() resolves. _load() ends with m.redraw(), which is async (queued for the next frame). Under slow first-load conditions — cold CDN, heavy DOM, additional _loaders like the one the emoji extension registers — the redraw can land after the 50ms timer fires, leaving the conditional <div class=\"TextEditor-editorContainer\"> out of the DOM when onbuild runs.

When that happens, this.\$('.TextEditor-editorContainer')[0] returns undefined and gets handed to whatever editor driver is being constructed:

this.attrs.composer.editor = this.buildEditor(this.\$('.TextEditor-editorContainer')[0]);

For the stock BasicEditorDriver, this produces a broken textarea. For Tiptap-based drivers (e.g. fof/rich-text), Tiptap v3 only calls its internal mount() if options.element is truthy, so the view is permanently null and the composer renders a non-functional toolbar with no editor underneath. The bug is intermittent because it depends on whether the redraw beats the timer.

Fix

Switch _load's trailing m.redraw() to m.redraw.sync() and drop the 50ms timer in oncreate. After m.redraw.sync() returns, the DOM has been updated to reflect loading = false, so the container element is present when onbuild runs. The .then callback runs on a microtask outside any Mithril event handler, so calling m.redraw.sync() from there is safe.

   oncreate(vnode) {
     super.oncreate(vnode);

-    this._load().then(() => {
-      setTimeout(this.onbuild.bind(this), 50);
-    });
+    this._load().then(this.onbuild.bind(this));
   }

   _load() {
     return Promise.all(this._loaders.map((loader) => loader())).then(() => {
       this.loading = false;
-      m.redraw();
+      m.redraw.sync();
     });
   }

Test plan

Repro using the script from the linked issue:

const orig = m.redraw;
m.redraw = function() { setTimeout(() => orig(), 300); };
m.redraw.sync = orig.sync;

• Before fix, with the slowdown applied: opening the composer produces a broken textarea (this.\$('.TextEditor-editorContainer')[0] is undefined).
• After fix, with the slowdown applied: composer opens cleanly, editor mounts, typing works.
• After fix, no slowdown: normal composer use is unchanged.

Closes #4612