fix: _refresh_hierarchy tolerates an empty screen stack (#147)

[hellices]

Fixes #147

Problem

Intermittent CI failures in tests/ui/test_ctx_switch.py (seen on PRs #140 and #146, ~20 min rerun each): textual.app.ScreenStackError: No screens on stack. The attempt-1 CI traceback pins the frame:

src/korvid/ui/app.py:1227 in on_resources_updated -> _refresh_hierarchy
src/korvid/ui/app.py:2390 -> screen = self.screen
textual/app.py:1641 -> ScreenStackError

A ResourcesUpdated message dispatched during app teardown lands after the screen stack is emptied; the unguarded App.screen read raises, run_test() re-raises it, and whichever test happened to be running fails.

Fix

Treat "no screen" as "no tree to refresh": catch ScreenStackError around the App.screen read in _refresh_hierarchy and return.

Test

test_refresh_hierarchy_survives_an_empty_screen_stack reproduces the teardown interleaving deterministically (App.screen property raising ScreenStackError) - RED before the fix, GREEN after.

Full gate (make check) green: 2828 passed.

[Copilot]

Pull request overview

Prevents teardown-time hierarchy refreshes from failing when Textual’s screen stack is empty.

Changes:

• Catch ScreenStackError during hierarchy refresh.
• Add a deterministic regression test for issue #147.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/korvid/ui/app.py	Safely skips refresh when no screen exists.
tests/ui/test_hierarchy_nav.py	Tests empty-screen-stack handling.

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[my-reviewer-agent]

이슈 #147의 flaky CI 원인(teardown 중 dispatch된 ResourcesUpdated가 빈 screen stack에서 App.screen을 읽어 ScreenStackError로 전체 테스트 실행을 실패시키는 문제)에 대한 정확하고 최소한의 수정입니다.

• _refresh_hierarchy에서 self.screen 읽기를 try/except ScreenStackError로 감싸고 조기 반환 — "화면 없음 = 열린 트리 없음" 의미가 자연스럽고, 기존의 isinstance(screen, HierarchyScreen) 가드와도 일관됩니다.
• 테스트가 property mock으로 teardown 인터리빙을 결정적으로 재현하며 RED→GREEN 검증됨.

Suggestion (비차단): on_resources_updated 경로의 다른 self.screen 접근 지점도 동일한 teardown 레이스에 노출될 수 있으니, 추후 같은 증상이 재발하면 핸들러 진입부에서 공통 가드를 고려해 보세요. 또한 테스트 말미의 assert True는 없어도 무방합니다.

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