(core) Fix flaky FormulaEditor test by adjusting focus behavior of FormulaEditor.

Summary:
The issue is in the app due to the possibility of subtle differences in
order of events. It's hard to trigger a wrong order, but the fix is
intended to make it impossible.

Test Plan: Running test with a bunch of iterations, to see how reliable it is

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: georgegevoian

Differential Revision: https://phab.getgrist.com/D3918

app/client/widgets/FormulaEditor.ts

@@ -375,7 +375,15 @@ export class FormulaEditor extends NewBaseEditor {
     }
     // Resize editor in case it is needed.
     this._formulaEditor.resize();
+
+    // This focus method will try to focus a textarea immediately and again on setTimeout. But
+    // other things may happen by the setTimeout time, messing up focus. The reason the immediate
+    // call doesn't usually help is that this is called on 'mousedown' before its corresponding
+    // focus/blur occur. We can do a bit better by restoring focus immediately after blur occurs.
     aceObj.focus();
+    const lis = dom.onElem(aceObj.textInput.getElement(), 'blur', e => { lis.dispose(); aceObj.focus(); });
+    // If no blur right away, clear the listener, to avoid unexpected interference.
+    setTimeout(() => lis.dispose(), 0);
   }
 }