fix: stop nested splits at parent merge joins

mdl/executor/cmd_microflows_show_helpers.go

@@ -783,10 +783,11 @@ func traverseFlowUntilMerge(
 		nestedMergeID := splitMergeMap[currentID]
 
 		trueFlow, falseFlow := findBranchFlows(flows)
+		nestedMergeID = resolveNestedMergeID(nestedMergeID, mergeID, trueFlow, falseFlow, flowsByOrigin)
 
 		// Empty-then swap: negate when true branch is empty but false branch has content.
 		// Skip when both branches go directly to merge (both empty).
-		if trueFlow != nil && falseFlow != nil && nestedMergeID != "" {
+		if trueFlow != nil && falseFlow != nil && nestedMergeID != "" && nestedMergeID == mergeID {
 			if trueFlow.DestinationID == nestedMergeID && falseFlow.DestinationID != nestedMergeID {
 				stmt = negateIfCondition(stmt)
 				trueFlow, falseFlow = falseFlow, trueFlow
@@ -938,6 +939,63 @@ func continueAfterNestedSplitJoin(
 	traverseFlowUntilMerge(ctx, joinID, parentMergeID, activityMap, flowsByOrigin, flowsByDest, splitMergeMap, visited, entityNames, microflowNames, lines, indent, sourceMap, headerLineCount, annotationsByTarget)
 }
 
+func resolveNestedMergeID(
+	nestedMergeID model.ID,
+	parentMergeID model.ID,
+	trueFlow *microflows.SequenceFlow,
+	falseFlow *microflows.SequenceFlow,
+	flowsByOrigin map[model.ID][]*microflows.SequenceFlow,
+) model.ID {
+	if nestedMergeID != "" && parentMergeID != "" && nestedMergeID != parentMergeID &&
+		canReachNode(parentMergeID, nestedMergeID, flowsByOrigin, make(map[model.ID]bool)) {
+		for _, flow := range []*microflows.SequenceFlow{trueFlow, falseFlow} {
+			if flow == nil {
+				continue
+			}
+			if flow.DestinationID == parentMergeID ||
+				canReachNode(flow.DestinationID, parentMergeID, flowsByOrigin, make(map[model.ID]bool)) {
+				return parentMergeID
+			}
+		}
+	}
+	if nestedMergeID != "" || parentMergeID == "" {
+		return nestedMergeID
+	}
+	for _, flow := range []*microflows.SequenceFlow{trueFlow, falseFlow} {
+		if flow == nil {
+			continue
+		}
+		if canReachNode(flow.DestinationID, parentMergeID, flowsByOrigin, make(map[model.ID]bool)) {
+			return parentMergeID
+		}
+	}
+	return ""
+}
+
+func canReachNode(
+	currentID model.ID,
+	targetID model.ID,
+	flowsByOrigin map[model.ID][]*microflows.SequenceFlow,
+	visited map[model.ID]bool,
+) bool {
+	if currentID == "" {
+		return false
+	}
+	if currentID == targetID {
+		return true
+	}
+	if visited[currentID] {
+		return false
+	}
+	visited[currentID] = true
+	for _, flow := range findNormalFlows(flowsByOrigin[currentID]) {
+		if canReachNode(flow.DestinationID, targetID, flowsByOrigin, visited) {
+			return true
+		}
+	}
+	return false
+}
+
 // traverseLoopBody traverses activities inside a loop body.
 // When sourceMap is non-nil, it also records line ranges for each activity node.
 func traverseLoopBody(

mdl/executor/cmd_microflows_traverse_test.go

@@ -654,6 +654,46 @@ func TestTraverseFlow_NestedTerminalGuardBranchSuppressesEmptyOuterElse(t *testi
 	}
 }
 
+func TestResolveNestedMergeID_UsesParentMergeBeforeDownstreamJoin(t *testing.T) {
+	flowsByOrigin := map[model.ID][]*microflows.SequenceFlow{
+		mkID("parent_merge"): {mkFlow("parent_merge", "downstream_join")},
+	}
+	trueFlow := mkFlow("nested_split", "parent_merge")
+	falseFlow := mkFlow("nested_split", "false_branch")
+
+	got := resolveNestedMergeID(
+		mkID("downstream_join"),
+		mkID("parent_merge"),
+		trueFlow,
+		falseFlow,
+		flowsByOrigin,
+	)
+
+	if got != mkID("parent_merge") {
+		t.Fatalf("nested split used downstream join %q, want parent merge %q", got, mkID("parent_merge"))
+	}
+}
+
+func TestResolveNestedMergeID_KeepsIndependentNestedJoin(t *testing.T) {
+	flowsByOrigin := map[model.ID][]*microflows.SequenceFlow{
+		mkID("nested_join"): {mkFlow("nested_join", "parent_merge")},
+	}
+	trueFlow := mkFlow("nested_split", "true_branch")
+	falseFlow := mkFlow("nested_split", "nested_join")
+
+	got := resolveNestedMergeID(
+		mkID("nested_join"),
+		mkID("parent_merge"),
+		trueFlow,
+		falseFlow,
+		flowsByOrigin,
+	)
+
+	if got != mkID("nested_join") {
+		t.Fatalf("nested split merge changed to %q, want local nested join %q", got, mkID("nested_join"))
+	}
+}
+
 // =============================================================================
 // collectErrorHandlerStatements
 // =============================================================================