4 files changed, 68 insertions, 8 deletions

diff --git a/src/nvim/tui/input.c b/src/nvim/tui/input.c
index 98dd7b4b45..744d306c06 100644
--- a/src/nvim/tui/input.c
+++ b/src/nvim/tui/input.c
@@ -160,12 +160,16 @@ void tinput_init(TermInput *input, Loop *loop)
   // initialize a timer handle for handling ESC with libtermkey
   uv_timer_init(&loop->uv, &input->timer_handle);
   input->timer_handle.data = input;
+
+  uv_timer_init(&loop->uv, &input->bg_query_timer);
+  input->bg_query_timer.data = input;
 }
 
 void tinput_destroy(TermInput *input)
 {
   map_destroy(int, &kitty_key_map);
   uv_close((uv_handle_t *)&input->timer_handle, NULL);
+  uv_close((uv_handle_t *)&input->bg_query_timer, NULL);
   rstream_may_close(&input->read_stream);
   termkey_destroy(input->tk);
 }
@@ -179,6 +183,7 @@ void tinput_stop(TermInput *input)
 {
   rstream_stop(&input->read_stream);
   uv_timer_stop(&input->timer_handle);
+  uv_timer_stop(&input->bg_query_timer);
 }
 
 static void tinput_done_event(void **argv)
@@ -474,6 +479,13 @@ static void tinput_timer_cb(uv_timer_t *handle)
   tinput_flush(input);
 }
 
+static void bg_query_timer_cb(uv_timer_t *handle)
+  FUNC_ATTR_NONNULL_ALL
+{
+  TermInput *input = handle->data;
+  tui_query_bg_color(input->tui_data);
+}
+
 /// Handle focus events.
 ///
 /// If the upcoming sequence of bytes in the input stream matches the termcode
@@ -660,6 +672,33 @@ static void handle_unknown_csi(TermInput *input, const TermKeyKey *key)
       }
     }
     break;
+  case 'n':
+    // Device Status Report (DSR)
+    if (nparams == 2) {
+      int args[2];
+      for (size_t i = 0; i < ARRAY_SIZE(args); i++) {
+        if (termkey_interpret_csi_param(params[i], &args[i], NULL, NULL) != TERMKEY_RES_KEY) {
+          return;
+        }
+      }
+
+      if (args[0] == 997) {
+        // Theme update notification
+        // https://github.com/contour-terminal/contour/blob/master/docs/vt-extensions/color-palette-update-notifications.md
+        // The second argument tells us whether the OS theme is set to light
+        // mode or dark mode, but all we care about is the background color of
+        // the terminal emulator. We query for that with OSC 11 and the response
+        // is handled by the autocommand created in _defaults.lua. The terminal
+        // may send us multiple notifications all at once so we use a timer to
+        // coalesce the queries.
+        if (uv_timer_get_due_in(&input->bg_query_timer) > 0) {
+          return;
+        }
+
+        uv_timer_start(&input->bg_query_timer, bg_query_timer_cb, 100, 0);
+      }
+    }
+    break;
   default:
     break;
   }
diff --git a/src/nvim/tui/input.h b/src/nvim/tui/input.h
index 4c2baf908e..f6aaff30de 100644
--- a/src/nvim/tui/input.h
+++ b/src/nvim/tui/input.h
@@ -32,6 +32,7 @@ typedef struct {
   TermKey *tk;
   TermKey_Terminfo_Getstr_Hook *tk_ti_hook_fn;  ///< libtermkey terminfo hook
   uv_timer_t timer_handle;
+  uv_timer_t bg_query_timer;  ///< timer used to batch background color queries
   Loop *loop;
   RStream read_stream;
   TUIData *tui_data;
diff --git a/src/nvim/tui/tui.c b/src/nvim/tui/tui.c
index 6e0012096b..603db5b891 100644
--- a/src/nvim/tui/tui.c
+++ b/src/nvim/tui/tui.c
@@ -241,16 +241,19 @@ void tui_handle_term_mode(TUIData *tui, TermMode mode, TermModeState state)
       tui->unibi_ext.sync = (int)unibi_add_ext_str(tui->ut, "Sync",
                                                    "\x1b[?2026%?%p1%{1}%-%tl%eh%;");
       break;
-    case kTermModeResizeEvents:
-      signal_watcher_stop(&tui->winch_handle);
-      tui_set_term_mode(tui, mode, true);
-      break;
     case kTermModeGraphemeClusters:
       if (!is_set) {
         tui_set_term_mode(tui, mode, true);
         tui->did_set_grapheme_cluster_mode = true;
       }
       break;
+    case kTermModeThemeUpdates:
+      tui_set_term_mode(tui, mode, true);
+      break;
+    case kTermModeResizeEvents:
+      signal_watcher_stop(&tui->winch_handle);
+      tui_set_term_mode(tui, mode, true);
+      break;
     }
   }
 }
@@ -320,6 +323,18 @@ static void tui_reset_key_encoding(TUIData *tui)
   }
 }
 
+/// Write the OSC 11 sequence to the terminal emulator to query the current
+/// background color.
+///
+/// The response will be handled by the TermResponse autocommand created in
+/// _defaults.lua.
+void tui_query_bg_color(TUIData *tui)
+  FUNC_ATTR_NONNULL_ALL
+{
+  out(tui, S_LEN("\x1b]11;?\x07"));
+  flush_buf(tui);
+}
+
 /// Enable the alternate screen and emit other control sequences to start the TUI.
 ///
 /// This is also called when the TUI is resumed after being suspended. We reinitialize all state
@@ -438,14 +453,13 @@ static void terminfo_start(TUIData *tui)
   // Enable bracketed paste
   unibi_out_ext(tui, tui->unibi_ext.enable_bracketed_paste);
 
-  // Query support for mode 2026 (Synchronized Output). Some terminals also
-  // support an older DCS sequence for synchronized output, but we will only use
-  // mode 2026.
+  // Query support for private DEC modes that Nvim can take advantage of.
   // Some terminals (such as Terminal.app) do not support DECRQM, so skip the query.
   if (!nsterm) {
     tui_request_term_mode(tui, kTermModeSynchronizedOutput);
-    tui_request_term_mode(tui, kTermModeResizeEvents);
     tui_request_term_mode(tui, kTermModeGraphemeClusters);
+    tui_request_term_mode(tui, kTermModeThemeUpdates);
+    tui_request_term_mode(tui, kTermModeResizeEvents);
   }
 
   // Don't use DECRQSS in screen or tmux, as they behave strangely when receiving it.
@@ -493,6 +507,10 @@ static void terminfo_start(TUIData *tui)
 /// Disable the alternate screen and prepare for the TUI to close.
 static void terminfo_stop(TUIData *tui)
 {
+  // Disable theme update notifications. We do this first to avoid getting any
+  // more notifications after we reset the cursor and any color palette changes.
+  tui_set_term_mode(tui, kTermModeThemeUpdates, false);
+
   // Destroy output stuff
   tui_mode_change(tui, NULL_STRING, SHAPE_IDX_N);
   tui_mouse_off(tui);
@@ -509,6 +527,7 @@ static void terminfo_stop(TUIData *tui)
   if (tui->did_set_grapheme_cluster_mode) {
     tui_set_term_mode(tui, kTermModeGraphemeClusters, false);
   }
+
   // May restore old title before exiting alternate screen.
   tui_set_title(tui, NULL_STRING);
   if (ui_client_exit_status == 0) {
diff --git a/src/nvim/tui/tui_defs.h b/src/nvim/tui/tui_defs.h
index bd99d6b0ad..5d6f027bf7 100644
--- a/src/nvim/tui/tui_defs.h
+++ b/src/nvim/tui/tui_defs.h
@@ -5,6 +5,7 @@ typedef struct TUIData TUIData;
 typedef enum {
   kTermModeSynchronizedOutput = 2026,
   kTermModeGraphemeClusters = 2027,
+  kTermModeThemeUpdates = 2031,
   kTermModeResizeEvents = 2048,
 } TermMode;