- Create new code block.
- Set language name to something unknown (e.g. "foozba'al").
- Enter some content below.
Observe the page remaining empty, but an inspector appears nonetheless of a piece of content that is no longer focusable (e.g. once clicking away from this, it's hard to recover).
When reviewing changes, the inserted wikitext, however, is as expected and does contain the source code block. Saving the changes and looking at the saved page also shows the content normally.
The bug is not limited to newly inserted nodes. It also happens when changing an existing code block's language. Or when starting the edit session with a page that already contains such code block.
This is especially problematic because the list of lexers can change sometimes and that makes existing blocks rather hard to fix.