feat(tui): use TermResponse event for OSC responses (#25868)

When the terminal emulator sends an OSC sequence to Nvim (as a response
to another OSC sequence that was first sent by Nvim), populate the OSC
sequence in the v:termresponse variable and fire the TermResponse event.
The escape sequence is also included in the "data" field of the
autocommand callback when the autocommand is defined in Lua.

This makes use of the already documented but unimplemented TermResponse
event. This event exists in Vim but is only fired when Vim receives a
primary device attributes response.

Fixes: https://github.com/neovim/neovim/issues/25856

2 files changed, 53 insertions, 0 deletions

diff --git a/src/nvim/api/ui.c b/src/nvim/api/ui.c
index b508a3ee94..99215f7b4f 100644
--- a/src/nvim/api/ui.c
+++ b/src/nvim/api/ui.c
@@ -16,6 +16,7 @@
 #include "nvim/api/ui.h"
 #include "nvim/autocmd.h"
 #include "nvim/channel.h"
+#include "nvim/eval.h"
 #include "nvim/event/loop.h"
 #include "nvim/event/wstream.h"
 #include "nvim/globals.h"
@@ -524,6 +525,32 @@ void nvim_ui_pum_set_bounds(uint64_t channel_id, Float width, Float height, Floa
   ui->pum_pos = true;
 }
 
+/// Tells Nvim when a terminal event has occurred.
+///
+/// The following terminal events are supported:
+///
+///   - "osc_response": The terminal sent a OSC response sequence to Nvim. The
+///                     payload is the received OSC sequence.
+///
+/// @param channel_id
+/// @param event Event name
+/// @param payload Event payload
+/// @param[out] err Error details, if any.
+void nvim_ui_term_event(uint64_t channel_id, String event, Object value, Error *err)
+  FUNC_API_SINCE(12) FUNC_API_REMOTE_ONLY
+{
+  if (strequal("osc_response", event.data)) {
+    if (value.type != kObjectTypeString) {
+      api_set_error(err, kErrorTypeValidation, "osc_response must be a string");
+      return;
+    }
+
+    const String osc_response = value.data.string;
+    set_vim_var_string(VV_TERMRESPONSE, osc_response.data, (ptrdiff_t)osc_response.size);
+    apply_autocmds_group(EVENT_TERMRESPONSE, NULL, NULL, false, AUGROUP_ALL, NULL, NULL, &value);
+  }
+}
+
 static void flush_event(UIData *data)
 {
   if (data->cur_event) {
diff --git a/src/nvim/tui/input.c b/src/nvim/tui/input.c
index 487f8b010e..1f659264c5 100644
--- a/src/nvim/tui/input.c
+++ b/src/nvim/tui/input.c
@@ -11,6 +11,7 @@
 #include "nvim/api/private/helpers.h"
 #include "nvim/ascii.h"
 #include "nvim/charset.h"
+#include "nvim/eval.h"
 #include "nvim/event/defs.h"
 #include "nvim/log.h"
 #include "nvim/macros.h"
@@ -479,6 +480,8 @@ static void tk_getkeys(TermInput *input, bool force)
           }
         }
       }
+    } else if (key.type == TERMKEY_TYPE_OSC) {
+      handle_osc_event(input, &key);
     }
   }
 
@@ -684,6 +687,29 @@ HandleState handle_background_color(TermInput *input)
   return kComplete;
 }
 
+static void handle_osc_event(TermInput *input, const TermKeyKey *key)
+{
+  assert(input);
+
+  const char *str = NULL;
+  if (termkey_interpret_string(input->tk, key, &str) == TERMKEY_RES_KEY) {
+    assert(str != NULL);
+
+    // Send an event to nvim core. This will update the v:termresponse variable and fire the
+    // TermResponse event
+    MAXSIZE_TEMP_ARRAY(args, 2);
+    ADD_C(args, STATIC_CSTR_AS_OBJ("osc_response"));
+
+    // libtermkey strips the OSC bytes from the response. We add it back in so that downstream
+    // consumers of v:termresponse can differentiate between OSC and CSI events.
+    StringBuilder response = KV_INITIAL_VALUE;
+    kv_printf(response, "\x1b]%s", str);
+    ADD_C(args, STRING_OBJ(cbuf_as_string(response.items, response.size)));
+    rpc_send_event(ui_client_channel_id, "nvim_ui_term_event", args);
+    kv_destroy(response);
+  }
+}
+
 static void handle_raw_buffer(TermInput *input, bool force)
 {
   HandleState is_paste = kNotApplicable;