Add ability to override cell size using env vars

tests/test_terminal.py

@@ -62,6 +62,27 @@ def capture_terminal_response(
                 assert get_cell_size() == (10, 20)
 
 
+def test_get_cell_size_env_variable_fallback() -> None:
+    if hasattr(get_cell_size, "_result"):
+        delattr(get_cell_size, "_result")
+
+    @contextmanager
+    def capture_terminal_response(
+        start_marker: str, end_marker: str, timeout: float | None = None
+    ) -> Iterator[SimpleNamespace]:
+        raise TimeoutError()
+
+    env = {"TEXTUAL_CELL_WIDTH": "12", "TEXTUAL_CELL_HEIGHT": "24"}
+    with patch("sys.__stdout__"):
+        with patch("textual_image._terminal.get_tiocgwinsz", side_effect=OSError()):
+            with patch("textual_image._terminal.capture_terminal_response", capture_terminal_response):
+                with patch.dict("os.environ", env, clear=True):
+                    result = get_cell_size()
+
+    assert result.width == 12
+    assert result.height == 24
+
+
 def test_get_cell_size_stdout_not_a_tty() -> None:
     if hasattr(get_cell_size, "_result"):
         delattr(get_cell_size, "_result")

textual_image/_terminal.py

@@ -1,6 +1,7 @@
 """Functionality to interact with the terminal."""
 
 import logging
+import os
 import sys
 from contextlib import contextmanager
 from types import SimpleNamespace
@@ -70,6 +71,16 @@ def get_cell_size() -> CellSize:
         except (TerminalError, TimeoutError) as e:
             logger.warning("Failed to get cell size via escape sequence, assuming VT340 sizes", exc_info=e)
 
+    if height == 0 or width == 0:
+        # Try environment variables (set by textual-serve for web terminals)
+        match os.environ:
+            case {
+                "TEXTUAL_CELL_WIDTH": str(width_str),
+                "TEXTUAL_CELL_HEIGHT": str(height_str),
+            } if width_str.isdigit() and height_str.isdigit():
+                width = int(width_str)
+                height = int(height_str)
+
     if height == 0 or width == 0:
         # Still didn't work, use VT340 sizes as default
         width = 10