feat(terminal): support theme update notifications (DEC mode 2031) (#31999)

9 files changed, 104 insertions, 0 deletions

diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index e6a1adf15b..23266d536f 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -337,6 +337,8 @@ TERMINAL
   unfocused terminal window will have no cursor at all (so there is nothing to
   highlight).
 • |jobstart()| gained the "term" flag.
+• The |terminal| will send theme update notifications when 'background' is
+  changed and DEC mode 2031 is enabled.
 
 TREESITTER
 
diff --git a/src/nvim/optionstr.c b/src/nvim/optionstr.c
index eac9ea02e0..93871905db 100644
--- a/src/nvim/optionstr.c
+++ b/src/nvim/optionstr.c
@@ -44,6 +44,7 @@
 #include "nvim/spellfile.h"
 #include "nvim/spellsuggest.h"
 #include "nvim/strings.h"
+#include "nvim/terminal.h"
 #include "nvim/types_defs.h"
 #include "nvim/vim_defs.h"
 #include "nvim/window.h"
@@ -532,6 +533,15 @@ const char *did_set_background(optset_T *args)
     check_string_option(&p_bg);
     init_highlight(false, false);
   }
+
+  // Notify all terminal buffers that the background color changed so they can
+  // send a theme update notification
+  FOR_ALL_BUFFERS(buf) {
+    if (buf->terminal) {
+      terminal_notify_theme(buf->terminal, dark);
+    }
+  }
+
   return NULL;
 }
 
diff --git a/src/nvim/terminal.c b/src/nvim/terminal.c
index ad343bad67..2ad5ac49ca 100644
--- a/src/nvim/terminal.c
+++ b/src/nvim/terminal.c
@@ -179,6 +179,8 @@ struct terminal {
     StringBuilder *send;  ///< When there is a pending TermRequest autocommand, block and store input.
   } pending;
 
+  bool theme_updates;  ///< Send a theme update notification when 'bg' changes
+
   bool color_set[16];
 
   char *selection_buffer;  /// libvterm selection buffer
@@ -193,6 +195,7 @@ static VTermScreenCallbacks vterm_screen_callbacks = {
   .movecursor = term_movecursor,
   .settermprop = term_settermprop,
   .bell = term_bell,
+  .theme = term_theme,
   .sb_pushline = term_sb_push,  // Called before a line goes offscreen.
   .sb_popline = term_sb_pop,
 };
@@ -1141,6 +1144,20 @@ bool terminal_running(const Terminal *term)
   return !term->closed;
 }
 
+void terminal_notify_theme(Terminal *term, bool dark)
+  FUNC_ATTR_NONNULL_ALL
+{
+  if (!term->theme_updates) {
+    return;
+  }
+
+  char buf[10];
+  ssize_t ret = snprintf(buf, sizeof(buf), "\x1b[997;%cn", dark ? '1' : '2');
+  assert(ret > 0);
+  assert((size_t)ret <= sizeof(buf));
+  terminal_send(term, buf, (size_t)ret);
+}
+
 static void terminal_focus(const Terminal *term, bool focus)
   FUNC_ATTR_NONNULL_ALL
 {
@@ -1259,6 +1276,10 @@ static int term_settermprop(VTermProp prop, VTermValue *val, void *data)
     invalidate_terminal(term, -1, -1);
     break;
 
+  case VTERM_PROP_THEMEUPDATES:
+    term->theme_updates = val->boolean;
+    break;
+
   default:
     return 0;
   }
@@ -1273,6 +1294,14 @@ static int term_bell(void *data)
   return 1;
 }
 
+/// Called when the terminal wants to query the system theme.
+static int term_theme(bool *dark, void *data)
+  FUNC_ATTR_NONNULL_ALL
+{
+  *dark = (*p_bg == 'd');
+  return 1;
+}
+
 /// Scrollback push handler: called just before a line goes offscreen (and libvterm will forget it),
 /// giving us a chance to store it.
 ///
diff --git a/src/nvim/vterm/screen.c b/src/nvim/vterm/screen.c
index c91c6fb84f..45bd5e2a27 100644
--- a/src/nvim/vterm/screen.c
+++ b/src/nvim/vterm/screen.c
@@ -784,6 +784,17 @@ static int resize(int new_rows, int new_cols, VTermStateFields *fields, void *us
   return 1;
 }
 
+static int theme(bool *dark, void *user)
+{
+  VTermScreen *screen = user;
+
+  if (screen->callbacks && screen->callbacks->theme) {
+    return (*screen->callbacks->theme)(dark, screen->cbdata);
+  }
+
+  return 1;
+}
+
 static int setlineinfo(int row, const VTermLineInfo *newinfo, const VTermLineInfo *oldinfo,
                        void *user)
 {
@@ -838,6 +849,7 @@ static VTermStateCallbacks state_cbs = {
   .settermprop = &settermprop,
   .bell = &bell,
   .resize = &resize,
+  .theme = &theme,
   .setlineinfo = &setlineinfo,
   .sb_clear = &sb_clear,
 };
diff --git a/src/nvim/vterm/state.c b/src/nvim/vterm/state.c
index 9e787acd9b..4ad07377de 100644
--- a/src/nvim/vterm/state.c
+++ b/src/nvim/vterm/state.c
@@ -819,6 +819,10 @@ static void set_dec_mode(VTermState *state, int num, int val)
     state->mode.bracketpaste = (unsigned)val;
     break;
 
+  case 2031:
+    settermprop_bool(state, VTERM_PROP_THEMEUPDATES, val);
+    break;
+
   default:
     DEBUG_LOG("libvterm: Unknown DEC mode %d\n", num);
     return;
@@ -894,6 +898,10 @@ static void request_dec_mode(VTermState *state, int num)
     reply = state->mode.bracketpaste;
     break;
 
+  case 2031:
+    reply = state->mode.theme_updates;
+    break;
+
   default:
     vterm_push_output_sprintf_ctrl(state->vt, C1_CSI, "?%d;%d$y", num, 0);
     return;
@@ -1387,6 +1395,7 @@ static int on_csi(const char *leader, const long args[], int argcount, const cha
 
     {
       char *qmark = (leader_byte == '?') ? "?" : "";
+      bool dark = false;
 
       switch (val) {
       case 0:
@@ -1403,6 +1412,13 @@ static int on_csi(const char *leader, const long args[], int argcount, const cha
         vterm_push_output_sprintf_ctrl(state->vt, C1_CSI, "%s%d;%dR", qmark, state->pos.row + 1,
                                        state->pos.col + 1);
         break;
+      case 996:
+        if (state->callbacks && state->callbacks->theme) {
+          if (state->callbacks->theme(&dark, state->cbdata)) {
+            vterm_push_output_sprintf_ctrl(state->vt, C1_CSI, "?997;%cn", dark ? '1' : '2');
+          }
+        }
+        break;
       }
     }
     break;
@@ -2268,6 +2284,9 @@ int vterm_state_set_termprop(VTermState *state, VTermProp prop, VTermValue *val)
   case VTERM_PROP_FOCUSREPORT:
     state->mode.report_focus = (unsigned)val->boolean;
     return 1;
+  case VTERM_PROP_THEMEUPDATES:
+    state->mode.theme_updates = (unsigned)val->boolean;
+    return 1;
 
   case VTERM_N_PROPS:
     return 0;
diff --git a/src/nvim/vterm/vterm_defs.h b/src/nvim/vterm/vterm_defs.h
index d0a8ba8814..9aa933bef0 100644
--- a/src/nvim/vterm/vterm_defs.h
+++ b/src/nvim/vterm/vterm_defs.h
@@ -86,6 +86,7 @@ typedef enum {
   VTERM_PROP_CURSORSHAPE,       // number
   VTERM_PROP_MOUSE,             // number
   VTERM_PROP_FOCUSREPORT,       // bool
+  VTERM_PROP_THEMEUPDATES,      // bool
 
   VTERM_N_PROPS,
 } VTermProp;
@@ -111,6 +112,7 @@ typedef struct {
   int (*settermprop)(VTermProp prop, VTermValue *val, void *user);
   int (*bell)(void *user);
   int (*resize)(int rows, int cols, void *user);
+  int (*theme)(bool *dark, void *user);
   int (*sb_pushline)(int cols, const VTermScreenCell *cells, void *user);
   int (*sb_popline)(int cols, VTermScreenCell *cells, void *user);
   int (*sb_clear)(void *user);
@@ -263,6 +265,7 @@ typedef struct {
   int (*settermprop)(VTermProp prop, VTermValue *val, void *user);
   int (*bell)(void *user);
   int (*resize)(int rows, int cols, VTermStateFields *fields, void *user);
+  int (*theme)(bool *dark, void *user);
   int (*setlineinfo)(int row, const VTermLineInfo *newinfo, const VTermLineInfo *oldinfo,
                      void *user);
   int (*sb_clear)(void *user);
diff --git a/src/nvim/vterm/vterm_internal_defs.h b/src/nvim/vterm/vterm_internal_defs.h
index 770f862ce3..d4d59867bf 100644
--- a/src/nvim/vterm/vterm_internal_defs.h
+++ b/src/nvim/vterm/vterm_internal_defs.h
@@ -119,6 +119,7 @@ struct VTermState {
     unsigned leftrightmargin:1;
     unsigned bracketpaste:1;
     unsigned report_focus:1;
+    unsigned theme_updates:1;
   } mode;
 
   VTermEncodingInstance encoding[4], encoding_utf8;
diff --git a/test/functional/terminal/tui_spec.lua b/test/functional/terminal/tui_spec.lua
index 3624a7bc2b..0a7dca8e6e 100644
--- a/test/functional/terminal/tui_spec.lua
+++ b/test/functional/terminal/tui_spec.lua
@@ -3309,6 +3309,32 @@ describe('TUI bg color', function()
       {3:-- TERMINAL --}                                    |
     ]])
   end)
+
+  it('sends theme update notifications when background changes #31652', function()
+    command('set background=dark') -- set outer Nvim background
+    local child_server = new_pipename()
+    local screen = tt.setup_child_nvim({
+      '--listen',
+      child_server,
+      '-u',
+      'NONE',
+      '-i',
+      'NONE',
+      '--cmd',
+      'colorscheme vim',
+      '--cmd',
+      'set noswapfile',
+    })
+    screen:expect({ any = '%[No Name%]' })
+    local child_session = n.connect(child_server)
+    retry(nil, nil, function()
+      eq({ true, 'dark' }, { child_session:request('nvim_eval', '&background') })
+    end)
+    command('set background=light') -- set outer Nvim background
+    retry(nil, nil, function()
+      eq({ true, 'light' }, { child_session:request('nvim_eval', '&background') })
+    end)
+  end)
 end)
 
 -- These tests require `tt` because --headless/--embed
diff --git a/test/unit/fixtures/vterm_test.c b/test/unit/fixtures/vterm_test.c
index 7522962a05..6744305960 100644
--- a/test/unit/fixtures/vterm_test.c
+++ b/test/unit/fixtures/vterm_test.c
@@ -345,6 +345,8 @@ static VTermValueType vterm_get_prop_type(VTermProp prop)
     return VTERM_VALUETYPE_INT;
   case VTERM_PROP_FOCUSREPORT:
     return VTERM_VALUETYPE_BOOL;
+  case VTERM_PROP_THEMEUPDATES:
+    return VTERM_VALUETYPE_BOOL;
 
   case VTERM_N_PROPS:
     return 0;