Due to technical limitations there are a few scenarios in which you can't switch from source mode to visual mode.
These are situations where syntax for a specific element (listed below) can span more that one line.
This syntax is unsupported because adding automatic indentation (using
:) to multiple lines would not work.
- comments containing table syntax
- comments containing template syntax
- comments containing extension syntax (including references)
You will still be able to compose these comments in source mode, although you should check the live preview to see if the automatic indentation causes any issues.
We hope to allow this syntax in the future but doing so will require a change to wikitext, which is being discussed in T230683.