The methods that are used in many places base became set_language_immediate and with_language_immediate, and the one with only a few call sites kept the intuitive name set_language.

Naming-wise, I think it's a bit odd to allow the name to be dominated by this term "immediate" that is just about a very subtle optimization. It'd be nice to have the frequently-used method keep the regular name set_language.

Could the buffer manage the may_block boolean automatically internally, by recording whether its syntax-highlighted text has ever been requested? That seems like it captures the intent behind the change, looking at the call sites. Alternatively, should we set the may_block state with some more explicit stateful API, where the editor tells the buffer that it is currently focused?