When `HTMLEditor` handles the design mode, `IMEContentObserver` observes the
`<body>` if there is. However, web apps may remove it. Then, we need to
recreate `IMEContentObserver` with emulating a focus move because it's difficult
to compute the difference between the old root and the new root (IME focus
notification sends all content to the parent, it's faster in most cases in this
situation). This was fixed in bug 1911010. However, there are remaining issues
after that.
When new `<body>` or the original `<body>` is connected again,
`IMEContentObserver` does not restart to observe the new `<body>`. So,
`IMEContentObserver` working differently after the `<body>` is even temporarily
removed. This make it harder to reproduce reported bugs.
Additionally, if the document element is removed, `IMEContentObserver` won't
be recreated until the document gets focus again even after new root and/or
`<body>` element is connected. This makes IME users inconvenient with such
tricky editors.
This patch makes `HTMLEditor::NotifyRootChanged` notifies `IMEStateManager`
of the editor root element change. Then, the new method of `IMEStateManager`
updates IME enabled state and recreate `IMEContentObserver` with emulating
a focus move.
Differential Revision: https://phabricator.services.mozilla.com/D220362
d9ceb68c92