When in a TinyMCE editor, in Source code mode, with the search panel open, keyboard navigation triggers an error, hence blocking the navigation in the form.

Problem found in 4.5, replicated in 5.1.

Steps to reproduce :

• Go to User Preferences page (/user/preferences.php), then click on "Edit profile".
• In the Description field, open the Source code (Tools > Source code).
• Open the search panel with the Ctrl-F shortcut.
• With the focus in the "Find" input, press on Tab to switch to the next field.
• The keyboard navigation doesn't work, and the error "Cannot read properties of null (reading 'focus') appears in the Console.

The problem seems to come from the tiny_html plugin, in the /lib/editor/tiny/plugins/html/amd/src/plugin.js file, where an event listener on the shadowRoot captures the keydown event.

The same problem occurs when using the Tab in the source code.