osc: update chapter marker positions when duration changes

Commit 6abb7e3 updates the markers when the chapters change, but it
doesn't update their relative position at the bar when the duration
changes.

This means that adding chapters to a live stream would result in
corresponding chapter markers which were static while the duration
changed and thus their positions became incorrect over time until the
OSC was reinitialized.

This is fixed by observing the duration property if chapters are present
and reinitializing the OSC when the duration changes.

The live_markers user option, which determines whether the duration
property is observed when there are chapters, has been added in order to
allow disabling this behaviour as calling request_init() frequently
might have some impact on low-end systems.

The impact of request_init() on render() was measured to increase from
1-1.5 ms to 2-3 ms on a 2010 MacBook Air, while the impact was neglible
on a 2016 Surface Book (increasing only to an average of 1.4 ms from
1.3 ms for n=1500 render cycles).

The live_markers option is enabled by default.

DOCS/man/osc.rst

@@ -375,6 +375,12 @@ Configurable Options
 
     Set to ``yes`` to reduce festivity (i.e. disable santa hat in December.)
 
+``livemarkers``
+    Default: yes
+
+    Update chapter markers positions on duration changes, e.g. live streams.
+    The updates are unoptimized - consider disabling it on very low-end systems.
+
 Script Commands
 ~~~~~~~~~~~~~~~
 

player/lua/osc.lua

@@ -48,6 +48,7 @@ local user_opts = {
     windowcontrols = "auto",    -- whether to show window controls
     windowcontrols_alignment = "right", -- which side to show window controls on
     greenandgrumpy = false,     -- disable santa hat
+    livemarkers = true,         -- update seekbar chapter markers on duration change
 }
 
 -- read options from config and command-line
@@ -1688,6 +1689,7 @@ function update_options(list)
     validate_user_opts()
     request_tick()
     visibility_mode(user_opts.visibility, true)
+    update_duration_watch()
     request_init()
 end
 
@@ -2592,13 +2594,39 @@ function enable_osc(enable)
     end
 end
 
+-- duration is observed for the sole purpose of updating chapter markers
+-- positions. live streams with chapters are very rare, and the update is also
+-- expensive (with request_init), so it's only observed when we have chapters
+-- and the user didn't disable the livemarkers option (update_duration_watch).
+function on_duration() request_init() end
+
+local duration_watched = false
+function update_duration_watch()
+    local want_watch = user_opts.livemarkers and
+                       (mp.get_property_number("chapters", 0) or 0) > 0 and
+                       true or false  -- ensure it's a boolean
+
+    if (want_watch ~= duration_watched) then
+        if want_watch then
+            mp.observe_property("duration", nil, on_duration)
+        else
+            mp.unobserve_property(on_duration)
+        end
+        duration_watched = want_watch
+    end
+end
+
 validate_user_opts()
+update_duration_watch()
 
 mp.register_event("shutdown", shutdown)
 mp.register_event("start-file", request_init)
 mp.observe_property("track-list", nil, request_init)
 mp.observe_property("playlist", nil, request_init)
-mp.observe_property("chapter-list", nil, request_init)
+mp.observe_property("chapter-list", nil, function()
+    update_duration_watch()
+    request_init()
+end)
 
 mp.register_script_message("osc-message", show_message)
 mp.register_script_message("osc-chapterlist", function(dur)