input: new option: --no-input-builtin-bindings

This is similar to [no-]input-default-bindings, but affects only
builtin bindings (while input-default-bindings affects anything which
config files can override, like scripting mp.add_key_binding).

Arguably, this is what input-default-binding should have always done,
however, it does not.

The reason we add a new option rather than repurpose/modify the
existing option is that it behaves differently enough to raise
concerns that it will break some use cases for existing users:
- The new option is only applied once on startup, while
  input-default-bindings can be modified effectively at runtime.
- They affects different sets of bindings, and it's possible that
  the set of input-default-bindings is useful enough to keep.

Implementation-wise, both options are trivial, so keeping one or the
other or both doesn't affect code complexity.

It could be argued that it would be useful to make the new option
also effective for runtime changes, however, this opens a can of
worms of how the bindings are stored beyond the initial setup.

TL;DR: it's impossible to differentiate correctly at runtime between
builtin bindings, and those added with mp.add_key_bindings.

The gist is that technically mpv needs/uses two binding "classes":
- weak/builtin bindings - lower priority than config files.
- "user" bindings - config files and "forced" runtime bindings.

input-default-bindings affects the first class trivially, but
input-builtin-bindings would not be able split this class further
at runtime without meaningful changes to a lot of delicate code.

So a new option it is. It should be useful to some libmpv clients
(players) which want to disable mpv's builtin bindings without
breaking mp.add_key_bindings for scripts.

Fixes #8809
(again. the previous fix 8edfe70b only improved the docs, while
now we're actually making the requested behavior possible)

DOCS/interface-changes.rst

@@ -43,6 +43,9 @@ Interface changes
     - add a `--watch-later-options` option to allow configuring which
       options quit-watch-later saves
     - make `current-window-scale` writeable and use it in the default input.conf
+    - add `--input-builtin-bindings` flag to control loading of built-in key
+      bindings during start-up (default: yes).
+
  --- mpv 0.33.0 ---
     - add `--d3d11-exclusive-fs` flag to enable D3D11 exclusive fullscreen mode
       when the player enters fullscreen.

DOCS/man/options.rst

@@ -3842,6 +3842,11 @@ Input
     using ``mp.add_key_binding`` (but not ``mp.add_forced_key_binding``).
     This might change in the future to exclude ``mp.add_key_binding``.
 
+``--no-input-builtin-bindings``
+    Disable loading of built-in key bindings during start-up. This option is
+    applied only during (lib)mpv initialization, and if used then it will not
+    be not possible to enable them later. May be useful to libmpv clients.
+
 ``--input-cmdlist``
     Prints all commands that can be bound to keys.
 

input/input.c

@@ -176,6 +176,7 @@ struct input_opts {
     int use_gamepad;
     int use_media_keys;
     int default_bindings;
+    int builtin_bindings;
     int enable_mouse_movements;
     int vo_key_input;
     int test;
@@ -190,6 +191,7 @@ const struct m_sub_options input_config = {
         {"input-keylist", OPT_PRINT(mp_print_key_list)},
         {"input-cmdlist", OPT_PRINT(mp_print_cmd_list)},
         {"input-default-bindings", OPT_FLAG(default_bindings)},
+        {"input-builtin-bindings", OPT_FLAG(builtin_bindings)},
         {"input-test", OPT_FLAG(test)},
         {"input-doubleclick-time", OPT_INT(doubleclick_time),
          M_RANGE(0, 1000)},
@@ -218,6 +220,7 @@ const struct m_sub_options input_config = {
         .enable_mouse_movements = 1,
         .use_media_keys = 1,
         .default_bindings = 1,
+        .builtin_bindings = 1,
         .vo_key_input = 1,
         .allow_win_drag = 1,
     },
@@ -1367,7 +1370,7 @@ void mp_input_load_config(struct input_ctx *ictx)
     // "Uncomment" the default key bindings in etc/input.conf and add them.
     // All lines that do not start with '# ' are parsed.
     bstr builtin = bstr0(builtin_input_conf);
-    while (builtin.len) {
+    while (ictx->opts->builtin_bindings && builtin.len) {
         bstr line = bstr_getline(builtin, &builtin);
         bstr_eatstart0(&line, "#");
         if (!bstr_startswith0(line, " "))