Automatically detect terminal background color

src/textual/_xterm_parser.py

@@ -14,7 +14,7 @@
 # When trying to determine whether the current sequence is a supported/valid
 # escape sequence, at which length should we give up and consider our search
 # to be unsuccessful?
-_MAX_SEQUENCE_SEARCH_THRESHOLD = 20
+_MAX_SEQUENCE_SEARCH_THRESHOLD = 24
 
 _re_mouse_event = re.compile("^" + re.escape("\x1b[") + r"(<?[\d;]+[mM]|M...)\Z")
 _re_terminal_mode_response = re.compile(
@@ -31,6 +31,10 @@
 """Sequence received when the terminal receives focus."""
 FOCUSOUT: Final[str] = "\x1b[O"
 """Sequence received when focus is lost from the terminal."""
+BG_COLOR: Final[str] = "\x1b]11;rgb:"
+"""Sequence received with information on terminal background color"""
+BG_COLOR_LEN: Final[int] = len(BG_COLOR) + 14
+"""Length of background color sequence"""
 
 _re_extended_key: Final = re.compile(r"\x1b\[(?:(\d+)(?:;(\d+))?)?([u~ABCDEFHPQRS])")
 
@@ -239,6 +243,19 @@ def reissue_sequence_as_keys(reissue_sequence: str) -> None:
                         bracketed_paste = False
                         break
 
+                    if sequence.startswith(BG_COLOR) and len(sequence) == BG_COLOR_LEN:
+                        rgb_str = sequence[len(BG_COLOR) :]
+                        rgb = rgb_str.split("/")
+                        if len(rgb) == 3:
+                            r = int(rgb[0], 16)
+                            g = int(rgb[1], 16)
+                            b = int(rgb[2], 16)
+                            self.debug_log(
+                                f"Detected BG_COLOR response {r:02x}/{g:02x}/{b:02x}"
+                            )
+                            on_token(events.BackgroundColor(r, g, b))
+                        break
+
                     if not bracketed_paste:
                         # Check cursor position report
                         if (

src/textual/app.py

@@ -3275,6 +3275,15 @@ async def _on_app_blur(self, event: events.AppBlur) -> None:
         self.app_focus = False
         self.screen.refresh_bindings()
 
+    def _is_dark_color(self, r: int, g: int, b: int) -> bool:
+        # perceived brightness formula
+        perceived_brightness = 0.2126 * r + 0.7152 * g + 0.0722 * b
+        return perceived_brightness < 128
+
+    async def _on_background_color(self, event: events.BackgroundColor) -> None:
+        """Background color detected"""
+        self.dark = self._is_dark_color(event.r & 0xFF, event.g & 0xFF, event.b & 0xFF)
+
     def _detach_from_dom(self, widgets: list[Widget]) -> list[Widget]:
         """Detach a list of widgets from the DOM.
 

src/textual/drivers/linux_driver.py

@@ -248,6 +248,7 @@ def on_terminal_resize(signum, stack) -> None:
         self.flush()
         self._key_thread = Thread(target=self._run_input_thread)
         send_size_event()
+        self.write("\x1b]11;?\x07")  # Detect background color
         self._key_thread.start()
         self._request_terminal_sync_mode_support()
         self._enable_bracketed_paste()

src/textual/events.py

@@ -707,3 +707,14 @@ def __init__(self, text: str, stderr: bool = False) -> None:
     def __rich_repr__(self) -> rich.repr.Result:
         yield self.text
         yield self.stderr
+
+
+@dataclass
+class BackgroundColor(Event, bubble=False):
+    """Internal event used when background color of the terminal is
+    detected
+    """
+
+    r: int
+    g: int
+    b: int

tests/test_xterm_parser.py

@@ -91,7 +91,7 @@ def test_cant_match_escape_sequence_too_long(parser):
     """The sequence did not match, and we hit the maximum sequence search
     length threshold, so each character should be issued as a key-press instead.
     """
-    sequence = "\x1b[123456789123456789123"
+    sequence = "\x1b[123456789123456789123456"
     events = list(parser.feed(sequence))
 
     # Every character in the sequence is converted to a key press