TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 5: refactor(gui_2): migrate L5786 render_text_viewer_window ced to Result[T] (Phase 5)

Extract _render_text_viewer_window_ced_result helper from the
TextEditor set_text/render try/except in render_text_viewer_window CED
branch. Legacy wrapper drains errors to app._last_request_errors per FR-BC-4
event-handler pattern.

[pre-audit] L5786 INTERNAL_BROAD_CATCH
[post-audit] V count: 6 -> 5 (L5786 removed)

src/gui_2.py

@@ -5738,12 +5738,12 @@ def render_text_viewer_window(app: App) -> None:
   elif tv_type in renderer._lang_map:
    if app._text_viewer_editor is None:
     app._text_viewer_editor = ced.TextEditor(); app._text_viewer_editor.set_read_only_enabled(True); app._text_viewer_editor.set_show_line_numbers_enabled(True)
-   try:
-    app._text_viewer_editor.set_text(app.text_viewer_content)
-    if app.perf_profiling_enabled: app.perf_monitor.start_component("_render_text_viewer_ced")
-    app._text_viewer_editor.render(f"##ced_{app.text_viewer_title}", imgui.ImVec2(-1, -1))
-    if app.perf_profiling_enabled: app.perf_monitor.end_component("_render_text_viewer_ced")
-   except Exception as e: imgui.text_colored(theme.get_color("status_error"), f"CED Error: {e}"); imgui.text_unformatted(app.text_viewer_content)
+   ced_result = _render_text_viewer_window_ced_result(app)
+   if not ced_result.ok:
+    if not hasattr(app, '_last_request_errors'): app._last_request_errors = []
+    app._last_request_errors.append(("_render_text_viewer_window_ced", ced_result.errors[0]))
+    imgui.text_colored(theme.get_color("status_error"), f"CED Error: {ced_result.errors[0].message}")
+    imgui.text_unformatted(app.text_viewer_content)
   else:
    with imscope.child("tv_scroll", -1, -1, True):
     if app.text_viewer_wrap: imgui.push_text_wrap_pos(imgui.get_content_region_avail().x)
@@ -7988,6 +7988,34 @@ def _render_operations_hub_external_editor_panel_result(app: "App") -> Result[bo
    original=e,
   )])
 
+
+def _render_text_viewer_window_ced_result(app: "App") -> Result[bool]:
+ """Drain-aware variant of L5786 render_text_viewer_window CED branch.
+
+ Extracts the TextEditor set_text/render try/except from
+ render_text_viewer_window CED branch into a Result-returning helper. On
+ success, returns Result(data=True). On exception, converts the exception
+ to ErrorInfo and returns Result(data=False, errors=[ErrorInfo]).
+
+ The legacy wrapper drains errors to app._last_request_errors (per FR-BC-4
+ event-handler drain pattern; data plane attribute).
+
+ [C: src/gui_2.py:render_text_viewer_window (L5786 legacy wrapper)]
+ """
+ try:
+  app._text_viewer_editor.set_text(app.text_viewer_content)
+  if app.perf_profiling_enabled: app.perf_monitor.start_component("_render_text_viewer_ced")
+  app._text_viewer_editor.render(f"##ced_{app.text_viewer_title}", imgui.ImVec2(-1, -1))
+  if app.perf_profiling_enabled: app.perf_monitor.end_component("_render_text_viewer_ced")
+  return Result(data=True)
+ except Exception as e:
+  return Result(data=False, errors=[ErrorInfo(
+   kind=ErrorKind.INTERNAL,
+   message=f"Text viewer CED render failed: {e}",
+   source="gui_2._render_text_viewer_window_ced_result",
+   original=e,
+  )])
+
 #endregion: Phase 5 Event Handler Result Helpers
 
 #endregion: MMA

tests/test_gui_2_result.py

@@ -1142,3 +1142,46 @@ def test_phase_5_l5380_render_operations_hub_external_editor_panel_result_failur
  err = result.errors[0]
  assert err.source == "gui_2._render_operations_hub_external_editor_panel_result"
  assert "ext editor render blew up" in err.message
+
+
+def test_phase_5_l5786_render_text_viewer_window_ced_result_success():
+ """
+ L5786 _render_text_viewer_window_ced_result returns Result.ok=True on success.
+
+ The helper wraps the TextEditor set_text/render try/except in
+ render_text_viewer_window. On success, returns Result(data=True).
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock, patch
+ app = MagicMock()
+ app.perf_profiling_enabled = False
+ app.text_viewer_content   = "line1\nline2\n"
+ app.text_viewer_title     = "test.txt"
+ app._text_viewer_editor   = MagicMock()
+ result = gui_2._render_text_viewer_window_ced_result(app)
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data is True
+
+
+def test_phase_5_l5786_render_text_viewer_window_ced_result_failure():
+ """
+ L5786 _render_text_viewer_window_ced_result returns Result.ok=False with ErrorInfo on failure.
+
+ When TextEditor render raises, the helper converts the exception to ErrorInfo
+ and returns Result(data=False, errors=[ErrorInfo]).
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock, patch
+ app = MagicMock()
+ app.perf_profiling_enabled = False
+ app.text_viewer_content   = "line1\nline2\n"
+ app.text_viewer_title     = "test.txt"
+ mock_editor = MagicMock()
+ mock_editor.set_text.side_effect = RuntimeError("ced set_text failed")
+ app._text_viewer_editor   = mock_editor
+ result = gui_2._render_text_viewer_window_ced_result(app)
+ assert not result.ok, f"Expected ok=False on failure, got data: {result.data}"
+ assert result.errors, "Expected at least one error on failure"
+ err = result.errors[0]
+ assert err.source == "gui_2._render_text_viewer_window_ced_result"
+ assert "ced set_text failed" in err.message