feat: add resize debounce to TUI app (TUI-001) and enable viewport tests (TUI-002) (#584)

- Add RESIZE_DEBOUNCE_S (200ms), _resize_timer, on_resize, _apply_resize to tui/app.py BernsteinApp matching dashboard.py pattern
- Remove @pytest.mark.skip from TestResizeDebounce — all 6 tests now pass
- TUI-002 viewport clipping already implemented in dashboard.py; all 11 viewport tests pass

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: chernistry

src/bernstein/tui/app.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import json
+import logging
 import os
 import time
 from pathlib import Path
@@ -35,6 +36,8 @@
 # Constants
 # ---------------------------------------------------------------------------
 
+logger = logging.getLogger(__name__)
+
 SERVER_URL = os.environ.get("BERNSTEIN_SERVER_URL", "http://localhost:8052")
 _POLL_INTERVAL: float = 2.0
 
@@ -109,6 +112,9 @@ class BernsteinApp(App[None]):
     TITLE = "Bernstein"
     CSS_PATH: ClassVar[str] = "styles.tcss"  # type: ignore[assignment]
 
+    #: Resize debounce delay in seconds (TUI-001).
+    RESIZE_DEBOUNCE_S: ClassVar[float] = 0.2
+
     BINDINGS: ClassVar[list[BindingType]] = [
         Binding("q", "quit", "Quit", show=False),
         Binding("r", "refresh", "Refresh", show=False),
@@ -145,6 +151,7 @@ def __init__(self, poll_interval: float = _POLL_INTERVAL) -> None:
         self._action_bar_visible = False
         self._current_rows: list[TaskRow] = []
         self._log_offsets: dict[str, int] = {}  # session_id → last-read byte offset
+        self._resize_timer: object | None = None  # debounce timer handle (TUI-001)
 
     # -- layout ---------------------------------------------------------------
 
@@ -177,6 +184,27 @@ def on_mount(self) -> None:
         self._load_historical_logs()
         self.set_interval(self._poll_interval, self.action_refresh)
 
+    def on_resize(self, event: object) -> None:
+        """Debounce terminal resize events to avoid layout crashes (TUI-001).
+
+        Args:
+            event: The Textual Resize event.
+        """
+        if self._resize_timer is not None:
+            self._resize_timer.stop()  # type: ignore[union-attr]
+        self._resize_timer = self.set_timer(
+            self.RESIZE_DEBOUNCE_S,
+            self._apply_resize,
+        )
+
+    def _apply_resize(self) -> None:
+        """Apply debounced resize with error protection (TUI-001)."""
+        self._resize_timer = None
+        try:
+            self.refresh(layout=True)
+        except Exception:
+            logger.debug("Layout calculation error during resize (ignored)", exc_info=True)
+
     # -- historical log loading -----------------------------------------------
 
     _MAX_HISTORICAL_LINES: ClassVar[int] = 200

tests/unit/test_tui_resize.py

@@ -1,7 +1,4 @@
-"""Tests for TUI-001: terminal resize debounce.
-
-Skipped: RESIZE_DEBOUNCE_S / _apply_resize not yet implemented on BernsteinApp.
-"""
+"""Tests for TUI-001: terminal resize debounce."""
 
 from __future__ import annotations
 
@@ -12,7 +9,6 @@
 from bernstein.tui.app import BernsteinApp
 
 
-@pytest.mark.skip(reason="TUI-001: resize debounce not yet implemented")
 class TestResizeDebounce:
     """Tests for the resize debounce mechanism in BernsteinApp (TUI-001)."""