fix(monitors): keep gated forecast band continuous (no zero-width pinch)

The freshness-gated forecast band collapsed to a zero-width point at the flat
anchor (lower=upper=baseline), rendering as an hourglass for a future next-update
window and as a band detached from history for an imminent one. Carry the
next-refresh tolerance across the whole forecast so the band stays a continuous
width that connects to the historical band; the mean line still holds flat at
baseline then steps. Display-only — no effect on anomaly evaluation.

TG-1131

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: aarthy-dk

testgen/common/monitor_forecast.py

@@ -120,11 +120,16 @@ def gated_forecast_prediction(
     # typed throughout (baseline is already a float).
     lower_tol = float(definition.lower_tolerance) if definition.lower_tolerance is not None else None
     upper_tol = float(definition.upper_tolerance) if definition.upper_tolerance is not None else None
+    # The band carries the next-refresh tolerance across the whole forecast, flat anchor included.
+    # Anchoring the band to the tolerance — rather than collapsing it to the baseline at the flat
+    # anchor — keeps it a continuous width that connects to the historical band and avoids a
+    # zero-width pinch (which renders as an hourglass for a future window, or a band detached from
+    # history for an imminent one). The mean line still holds flat at baseline, then steps.
     return {
         "method": "predict",
         "mean": {flat_anchor: baseline, window_end: next_refresh_mean},
-        "lower_tolerance": {flat_anchor: baseline, window_end: lower_tol},
-        "upper_tolerance": {flat_anchor: baseline, window_end: upper_tol},
+        "lower_tolerance": {flat_anchor: lower_tol, window_end: lower_tol},
+        "upper_tolerance": {flat_anchor: upper_tol, window_end: upper_tol},
     }
 
 

tests/unit/ui/test_monitors_dashboard.py

@@ -135,10 +135,12 @@ def test_gated_prediction_anchors_at_now_when_window_started():
     mean = {str(NOW_MS + 3 * HOUR): 1200.0, str(NOW_MS + 27 * HOUR): 1300.0}
     result = gated_forecast_prediction(_gated_def(mean=mean), window, LAST_RUN)
     assert result["method"] == "predict"
+    # mean holds flat at baseline then steps to the next-refresh forecast
     assert result["mean"] == {NOW_MS: 1000.0, NOW_MS + 3 * HOUR: 1200.0}
-    # tolerances coerced from VARCHAR to float
-    assert result["lower_tolerance"] == {NOW_MS: 1000.0, NOW_MS + 3 * HOUR: 950.0}
-    assert result["upper_tolerance"] == {NOW_MS: 1000.0, NOW_MS + 3 * HOUR: 1400.0}
+    # band carries the next-refresh tolerance across the whole forecast (continuous width, no
+    # zero-width pinch at the flat anchor); coerced from VARCHAR to float
+    assert result["lower_tolerance"] == {NOW_MS: 950.0, NOW_MS + 3 * HOUR: 950.0}
+    assert result["upper_tolerance"] == {NOW_MS: 1400.0, NOW_MS + 3 * HOUR: 1400.0}
 
 
 def test_gated_prediction_anchors_at_window_start_when_future():