fix(UX-001): reconcile preview validation behavior

app/pages/backtests.py

@@ -289,6 +289,34 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
                         row_key="date",
                     ).classes("w-full")
 
+        def validate_current_scenario() -> str | None:
+            try:
+                service.run_read_only_scenario(
+                    symbol=str(symbol_input.value or ""),
+                    start_date=parse_iso_date(start_input.value, "Start date"),
+                    end_date=parse_iso_date(end_input.value, "End date"),
+                    template_slug=str(template_select.value or ""),
+                    underlying_units=float(units_input.value or 0.0),
+                    loan_amount=float(loan_input.value or 0.0),
+                    margin_call_ltv=float(ltv_input.value or 0.0),
+                )
+            except (ValueError, KeyError) as exc:
+                return str(exc)
+            except Exception:
+                logger.exception(
+                    "Backtest preview failed for workspace=%s symbol=%s start=%s end=%s template=%s units=%s loan=%s margin_call_ltv=%s",
+                    workspace_id,
+                    symbol_input.value,
+                    start_input.value,
+                    end_input.value,
+                    template_select.value,
+                    units_input.value,
+                    loan_input.value,
+                    ltv_input.value,
+                )
+                return "Backtest preview failed. Please verify the scenario inputs and try again."
+            return None
+
         def refresh_workspace_seeded_units() -> None:
             validation_label.set_text("")
             entry_spot, entry_error = derive_entry_spot()
@@ -304,7 +332,13 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
                 entry_spot_hint.set_text("Entry spot unavailable until the scenario dates are valid.")
             render_seeded_summary(entry_spot=entry_spot, entry_spot_error=entry_error)
             if entry_error:
+                validation_label.set_text(entry_error)
                 render_result_state("Scenario validation failed", entry_error, tone="warning")
+                return
+            validation_error = validate_current_scenario()
+            if validation_error:
+                validation_label.set_text(validation_error)
+                render_result_state("Scenario validation failed", validation_error, tone="warning")
             else:
                 mark_results_stale()
 
@@ -313,7 +347,14 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
             entry_spot, entry_error = derive_entry_spot()
             render_seeded_summary(entry_spot=entry_spot, entry_spot_error=entry_error)
             if entry_error:
+                entry_spot_hint.set_text("Entry spot unavailable until the scenario dates are valid.")
+                validation_label.set_text(entry_error)
                 render_result_state("Scenario validation failed", entry_error, tone="warning")
+                return
+            validation_error = validate_current_scenario()
+            if validation_error:
+                validation_label.set_text(validation_error)
+                render_result_state("Scenario validation failed", validation_error, tone="warning")
             else:
                 mark_results_stale()
 

app/pages/event_comparison.py

@@ -204,11 +204,12 @@ def _render_event_comparison_page(workspace_id: str | None = None) -> None:
 
             template_slugs = selected_template_slugs()
             if not template_slugs:
-                message = "Select at least one strategy template."
+                template_slugs = tuple(service.default_template_selection(str(option["slug"])))
-                metadata_label.set_text(f"Preset: {option['label']} — {option['description']}")
+                syncing_controls["value"] = True
-                scenario_label.set_text(message)
+                try:
-                render_selected_summary(entry_spot=None, entry_spot_error=message)
+                    template_select.value = list(template_slugs)
-                return message
+                finally:
+                    syncing_controls["value"] = False
 
             try:
                 preview_units = float(units_input.value or 0.0)
@@ -260,12 +261,15 @@ def _render_event_comparison_page(workspace_id: str | None = None) -> None:
         def render_report() -> None:
             validation_label.set_text("")
             result_panel.clear()
+            option = preset_lookup.get(str(preset_select.value or ""))
             template_slugs = selected_template_slugs()
-            if not template_slugs:
+            if option is not None and not template_slugs:
-                message = "Select at least one strategy template."
+                template_slugs = tuple(service.default_template_selection(str(option["slug"])))
-                validation_label.set_text(message)
+                syncing_controls["value"] = True
-                render_result_state("Scenario validation failed", message, tone="warning")
+                try:
-                return
+                    template_select.value = list(template_slugs)
+                finally:
+                    syncing_controls["value"] = False
             try:
                 report = service.run_read_only_comparison(
                     preset_slug=str(preset_select.value or ""),
@@ -406,6 +410,7 @@ def _render_event_comparison_page(workspace_id: str | None = None) -> None:
             validation_label.set_text("")
             preview_error = refresh_preview(reset_templates=True, reseed_units=True)
             if preview_error:
+                validation_label.set_text(preview_error)
                 render_result_state("Scenario validation failed", preview_error, tone="warning")
             else:
                 mark_results_stale()
@@ -416,6 +421,7 @@ def _render_event_comparison_page(workspace_id: str | None = None) -> None:
             validation_label.set_text("")
             preview_error = refresh_preview()
             if preview_error:
+                validation_label.set_text(preview_error)
                 render_result_state("Scenario validation failed", preview_error, tone="warning")
             else:
                 mark_results_stale()