scripting: don't observe properties with type nil

mp.observe_property('foo', nil, ...) calls the handler at least 2 times
on each playlist change even when the property doesn't change. This is
dangerous because if you haven't read observe_property's documentation
in a long time this is easy to forget, and you can end up using it for
handlers that are computationally expensive or that cause unintended
side effects.

Therefore, this commit discourages its use more explicitly in the
documentation, and replaces its usages in scripts.

For console.lua, observing focused with type nil leads to calling
mp.osd_message('') when changing file while playing in the terminal with
the console disabled. I don't notice issues from this, but it's safer to
avoid it.

For playlist and track-list this doesn't really matter since they
trigger multiple changes on each new file anyway, but changing it can
avoid encouraging people to imitate the code.

One usage of none in stats.lua is kept because according to b9084dfd47
it is a hack to replicate the deprecated tick event.

1 files changed, 3 insertions, 3 deletions

diff --git a/DOCS/man/lua.rst b/DOCS/man/lua.rst
index 1ea612a0be..6a18e60e1d 100644
--- a/DOCS/man/lua.rst
+++ b/DOCS/man/lua.rst
@@ -426,9 +426,9 @@ The ``mp`` module is preloaded, although it can be loaded manually with
     This depends on the property, and it's a valid feature request to ask for
     better update handling of a specific property.
 
-    If the ``type`` is ``none`` or ``nil``, sporadic property change events are
-    possible. This means the change function ``fn`` can be called even if the
-    property doesn't actually change.
+    If the ``type`` is ``none`` or ``nil``, the change function ``fn`` will be
+    called sporadically even if the property doesn't actually change. You should
+    therefore avoid using these types.
 
     You always get an initial change notification. This is meant to initialize
     the user's state to the current value of the property.