Toggle Just-in-time Lock mode.
Turn Just-in-time Lock mode on if and only if ARG is non-nil.
Enable it automatically by customizing group `font-lock'.
When Just-in-time Lock mode is enabled, fontification is different in the
- Demand-driven buffer fontification triggered by Emacs C code.
This means initial fontification of the whole buffer does not occur.
Instead, fontification occurs when necessary, such as when scrolling
through the buffer would otherwise reveal unfontified areas. This is
useful if buffer fontification is too slow for large buffers.
- Stealthy buffer fontification if `jit-lock-stealth-time' is non-nil.
This means remaining unfontified areas of buffers are fontified if Emacs has
been idle for `jit-lock-stealth-time' seconds, while Emacs remains idle.
This is useful if any buffer has any deferred fontification.
- Deferred context fontification if `jit-lock-contextually' is
non-nil. This means fontification updates the buffer corresponding to
true syntactic context, after `jit-lock-context-time' seconds of Emacs
idle time, while Emacs remains idle. Otherwise, fontification occurs
on modified lines only, and subsequent lines can remain fontified
corresponding to previous syntactic contexts. This is useful where
strings or comments span lines.
Stealth fontification only occurs while the system remains unloaded.
If the system load rises above `jit-lock-stealth-load' percent, stealth
fontification is suspended. Stealth fontification intensity is controlled via the variable `jit-lock-stealth-nice'.
Non-nil means Just-in-time Lock mode is active.