Automatically detect terminal background color

Add support for automatically detecting the background color of the
terminal using operating system command escape sequence. The response is
then parsed and used to determine if the background is light or dark.

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 +243,17 @@ 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 +707,13 @@ 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