In CONTRIBUTE.md's Setup section, users are instructed to use poetry shell to activate the virtual environment.

Poetry released a major upgrade to 2.0.0 in early January. This introduced a breaking change where the poetry shell command was removed completely.

The official documentation now recommends either passing the command poetry env activate to the shell, or installing the poetry-plugin-shell to add this command back to Poetry.

Poetry itself is a bit vague on the issue when it mentions none of these directly in response to poetry shell.

~/s/textual (main)> poetry shell

The command "shell" does not exist.

Did you mean one of these?
    self show
    self sync
    show
    self add
    self install
    install
    self lock
    help

This will need to be added to the documentation rather than replacing the old command, as not all OS have added the new version of Poetry to their stable repos yet (Debian Bookworm naturally being one that I've found at least).

ETA: There's also a fix to an issue for bash/fish/zsh shells awaiting the 2.1.0 release as currently poetry env activate doesn't act like old poetry shell where it created a virtual environment for you if one did not already exist (have to poetry env use as described on official website first).