What is the problem you are trying to solve?
Very often I create fragments with only HTML, CSS (and maybe json) code but without JS.
Also “preview” in my opinion is ofter unuseful because theme/css extension is not loaded in this environment, so no colors or other styles are not available in this pane.
What is your project about? (e. g. Intranet, Partner Portal, Enterprise Website, etc)
ALL
What is your proposed solution? (optional)
Could be useful to be able to dinamically resize panes to have bigger space for HTML and CSS and less for JS and preview when not needed.
Very often could be useful also to have JSON pane in the same tab of CODE panes…
So the perfect solution could be not only resize panes but also choose what panes I’d need and maybe re-arrange them.