Core

For testing:
AFAIK within the core, only the t3editor and the extension manager (edit files) uses the codemirror library ATM. There might be some Extensions, which might use the library too.

I had a closer look at it. Yes, t3editor and editing files should be the parts that need to be migrated.

Since the architecture of codemirror changed it's not that simply changing the codemirror-sources used. But the new architecture looks a lot simpler and more powerful to me, so I also see some value for cleaning up parts of the code during this migration.

• syntax-highlighting for typoscript: The original tokenizer/parser was written based upon a JavaScript-highlighter. I think we might use a similar highlighter shipped with codemirror as a basis and start to reimplement it. Architecture is much simpler now, and there are much more highlighters already shipped wiht codemirror. We might even want to re-contribute our highlighter for typoscript into the official distribution.
• code-completion: Haven't tried how much of our current approach can be left as-is as a first step. The functions triggering the completion need to be changed a bit, but fetching the completion etc. should be possible to leave unchanged for now.

After that change is done we could, as a separate task, take a look whether the several divs, iframe etc. for the editor are still needed and how we could maybe reduce that overhead. There is also quite a bit of prototype-code involved.

3.0 released. We might want to consider going directly to the 3.0-tree.
If so, somebody please update the issue-title.