Focus tracking for input on web is janky

When running an `eframe` app on web, sometimes focus is not properly assigned to the hidden text `` (referred to as "text agent" from this point on) which results in strange behavior.

So far we've observed:
- Selecting text in a mutable text field on _any_ platform doesn't focus the text agent at all
- _Clicking_ on a mutable text field sometimes doesn't work on iOS, MacOS, and Android

Start by reproducing the focus failures in an eframe app on web and the listed mobile platforms, then inspect the hidden text input described as the "text agent" and the mutable text-field focus path. Done means text selection and clicking reliably focus the text agent across the reported platforms.

Written by the indexing model from the issue text.