Use os.get_terminal_size as the primary

[wsxrdv]

The function shutil.get_terminal_size returns environment variables COLUMNS and LINES when those exist. But this information is stale after resizing the terminal. First try to retrieve from os.get_terminal_size and only after fall back to shutil.

Please review the following checklist.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[wsxrdv]

Previous version of src/textual/drivers/linux_driver.py had os.get_terminal_size but was changed by d1235e0 All other changes I found were directly adding the shutil version.

[wsxrdv]

This change allows proper reactivity to resize on Ubuntu inside tmux. One can also test these changes using the following script while resizing:

import os
import shutil
import time

while True:
  # os.get_terminal_size changes with resize
  # shutil.get_terminal_size uses env variables (is constant)
  print(f"{os.get_terminal_size() = } {shutil.get_terminal_size() = }")
  time.sleep(1)

[wsxrdv]

The Rich project also uses os.get_terminal_size: https://github.com/Textualize/rich/blob/ea9d4db5d84b4e834979304e3053bf757daae322/rich/console.py#L1030

[wsxrdv]

I tried to dig deeper:

• Debian 13 seems that neither COLUMNS nor LINES are present in Python os.environ and thus shutil is giving always the correct result.
• Jupyterlab shell with tmux run inside from a tensorflow/tensorflow:latest-gpu docker seems to have both COLUMNS and LINES in the os.environ and thus shutil gives the wrong result.
• Possible workaround (without changing textual) is removing COLUMNS and LINES from os.environ before starting the app:

for d in ["LINES", "COLUMNS"]:
    if d in os.environ:
        del os.environ[d]