Summary

Avoids rerendering when the markdown preview pane is hidden.

Fixes #9891.

Testing

1. Enable the CodeMirror 5 markdown editor
2. Paste ten copies of the Project Gutenberg Ebook of Frankenstein into a note.
3. Hide the markdown preview pane
4. Add text near the end of the document. Verify that the UI doesn't freeze after a brief pause in typing.
   • Without this pull request, on my computer, the UI freezes for a few seconds after a pause in typing. When this happens, if I try to continue typing, nothing new shows up until the UI unfreezes.
5. Repeat steps 1-4 for the CodeMirror 6 markdown editor.

This has been tested successfully on Ubuntu 23.10.