[baixinghao]

Summary

The @ picker listed directories with a trailing slash, but accepting one only replaced the token with @dir/, which immediately re-triggered completion on the directory's children. There was no way to commit a directory itself as a reference — users were forced to drill in and pick a file.

Accepting a directory completion now commits the reference: the caret moves past (or inserts) a separating space after @dir/, leaving the @ token so the picker closes instead of re-opening on children. This implements the "trailing space" option proposed in the issue discussion. Drilling into a directory still works by typing its path — every keystroke re-narrows the flat, recursive suggestion list.

Fixes #464.

Changes

• src/tau_coding/tui/app.py — action_accept_completion: when the accepted completion is a FILE_REFERENCE ending with / (a directory), reuse an existing following whitespace character or insert a space, and place the caret after it. File completions, shell-path completions (!cat …), and all picker-screen accept dispatch are unchanged.
• tests/test_tui_app.py — three new regression tests (see below).
• website/content/guides/tui.md — the "Referencing files with @" section now documents that directories are shown with a trailing / and that inserting one commits it as a directory reference.

Tests

New tests in tests/test_tui_app.py (Textual run_test + FakeSession, matching neighboring completion tests):

• test_tui_app_accepts_directory_reference_completion — Tab on @src/ commits inspect @src/ , closes the picker, and deleting the space re-opens child completion (drill-in by typing still works).
• test_tui_app_accepts_directory_reference_before_existing_space — mid-text accept reuses the existing following space instead of doubling it, and parks the caret after it.
• test_tui_app_submits_committed_directory_reference — Tab then Enter submits inspect @src/ to the session end to end.

Verification

• Reproduced first: the new tests fail on unmodified main (assert 'inspect @src/' == 'inspect @src/ '), then pass with the fix.
• uv run pytest tests/test_tui_autocomplete.py tests/test_commands.py — 88 passed.
• uv run pytest tests/test_tui_app.py -k "completion or mention or footer_hints or directory_reference or file_reference or shell_path" — 31 passed.
• Full tests/test_tui_app.py on this branch: 15 failed / 438 passed; on the stashed clean baseline: 13 failed / 437 passed. The differing tests (test_session_sidebar_lists_active_system_prompt_files, two login-picker tests) fail or pass flakily on both revisions and in isolation, and are unrelated to completion handling (sidebar rendering, login picker timing); the 12 remaining failures are identical pre-existing Windows failures on both revisions.
• uv run ruff check / uv run ruff format --check on the changed files — clean; uv run mypy src/tau_coding/tui/app.py — no issues.
• prettier --check content/guides/tui.md — clean.

Notes for reviewers

• The issue discussion also floated a second layer — expanding @dir/ into the directory's contents at submit time. Tau currently does not expand @ references at all (the path is sent as prompt text and the agent uses its tools), so there is no resolver to extend; this PR is scoped to the picker. If inline directory expansion is wanted later, it can be built separately.
• A side effect to be aware of: repeated Tab no longer walks into nested directories (each accepted directory commits). The completion list is flat and recursive with substring matching, so typing @login still finds src/components/auth/login.py directly, and typing a directory prefix by hand still lists its children.