1. The exact intention of this feature is what you see and report. It's intended to work only where that Template is used, otherwise, you can add the Code to Toolset > Layouts CSS and JS or your theme
2. I understand this is the reason you use Layouts, which is the correct thing to do.
As you might have noticed, meanwhile you also can choose if CSS/JS shall be applied site-wide (even if no Layouts is used) or not.
3. Search and find does work in that editor with a simple cmd+f, as in any browser, so that can be used as a "workaround" to the issue you currently mention about finding specific lines of code.
4. The solution proposed is great, but it requires a proper handling of the cascade in the tabs. Will tabs to your right are "bottom" of the CSS cascade, or will we allow the cascade to be customized even?
The idea, however, is nice, for large sites - I also see the organizational issue in a normal theme's Stylesheet, for which we then add comments and other meaningful code extensions so to find the code quickly (again with simple cmd+f in a text editor or in the online editor)
I understand it would give a nice organization to the code and more power, but it is a larger project than it seems.
Especially the UX is a problematic question and how to enqueue which tab at what point.
More, you would have a better look with 2 new menus, probably, as otherwise, you will have a Tab for JS and CSS and several sub-tabs for each (and growing) chunk of Code.
Themes Style sheet work the same, and I would recommend to rather look out for a Plugin that focuses on this, as to add this to Layouts core since the main focus on Layouts is to build the front end, not the backend.
Note that interestingly almost all plugins out there handling CSS are not maintained anymore, speaking for the less and less need of developers on this kind of features.
I am not sure this is the best feature to add to Toolset, after all, for a lot of users.
If you disagree please let me know, I can file the request and see how many people would like this as well
For that, I would need as many details as possible about your ideas on how to solve the above-illustrated issues with it.