csl.api: Fix GsfSemanticLayer after 0ba2e7a

The implementation of GsfSemanticLayer#firstSequenceElement returns
wrong/out-of-range values for the first element. This in turn causes
tooltip/rendering errors when the sequence is used/positioned.

Closes: apache#8102

ide/csl.api/src/org/netbeans/modules/csl/editor/semantic/GsfSemanticLayer.java

@@ -226,7 +226,7 @@ public AttributeSet getAttributes() {
      * @param offset The offset in the document.
      * @return the index of the first SequenceElement that is left to the offset
      */
-    private static int firstSequenceElement(List<SequenceElement> l, int offset) {
+    static int firstSequenceElement(List<SequenceElement> l, int offset) {
         int low = 0;
         int high = l.size() - 1;
 
@@ -235,12 +235,19 @@ private static int firstSequenceElement(List<SequenceElement> l, int offset) {
             SequenceElement midVal = l.get(mid);
             int cmp = midVal.range().getStart() - offset;
 
-            if (cmp < 0)
+            if (cmp == 0) {
+                return mid;
+            } else if (low == high) {
+                if (mid > 0 && cmp >= 0) {
+                    return mid - 1;
+                } else {
+                    return low;
+                }
+            } else if (cmp < 0) {
                 low = mid + 1;
-            else if (cmp > 0)
+            } else if (cmp > 0) {
                 high = mid - 1;
-            else
-                return mid;
+            }
         }
         return low;
     }

ide/csl.api/test/unit/src/org/netbeans/modules/csl/editor/semantic/GsfSemanticLayerTest.java

@@ -0,0 +1,64 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.netbeans.modules.csl.editor.semantic;
+
+import java.util.List;
+import org.junit.Test;
+import org.netbeans.modules.csl.api.ColoringAttributes;
+import org.netbeans.modules.csl.api.OffsetRange;
+import org.netbeans.modules.csl.core.Language;
+
+import static org.junit.Assert.assertEquals;
+
+
+public class GsfSemanticLayerTest {
+
+    @Test
+    public void testFirstSequenceElement() {
+        List<SequenceElement> elements = List.of(
+                new SequenceElement(new Language("text/x-dummy"), new OffsetRange(10, 20), ColoringAttributes.empty()),
+                new SequenceElement(new Language("text/x-dummy"), new OffsetRange(30, 40), ColoringAttributes.empty()),
+                new SequenceElement(new Language("text/x-dummy"), new OffsetRange(50, 60), ColoringAttributes.empty())
+        );
+
+        assertEquals(0, GsfSemanticLayer.firstSequenceElement(elements, -1));
+        assertEquals(0, GsfSemanticLayer.firstSequenceElement(elements, 0));
+        assertEquals(0, GsfSemanticLayer.firstSequenceElement(elements, 5));
+        assertEquals(0, GsfSemanticLayer.firstSequenceElement(elements, 9));
+        assertEquals(0, GsfSemanticLayer.firstSequenceElement(elements, 10));
+        assertEquals(0, GsfSemanticLayer.firstSequenceElement(elements, 15));
+        assertEquals(0, GsfSemanticLayer.firstSequenceElement(elements, 19));
+        assertEquals(0, GsfSemanticLayer.firstSequenceElement(elements, 20));
+        assertEquals(0, GsfSemanticLayer.firstSequenceElement(elements, 25));
+        assertEquals(0, GsfSemanticLayer.firstSequenceElement(elements, 29));
+        assertEquals(1, GsfSemanticLayer.firstSequenceElement(elements, 30));
+        assertEquals(1, GsfSemanticLayer.firstSequenceElement(elements, 35));
+        assertEquals(1, GsfSemanticLayer.firstSequenceElement(elements, 41));
+        assertEquals(1, GsfSemanticLayer.firstSequenceElement(elements, 49));
+        assertEquals(2, GsfSemanticLayer.firstSequenceElement(elements, 50));
+        assertEquals(2, GsfSemanticLayer.firstSequenceElement(elements, 59));
+        assertEquals(2, GsfSemanticLayer.firstSequenceElement(elements, 60));
+        assertEquals(2, GsfSemanticLayer.firstSequenceElement(elements, 61));
+        assertEquals(2, GsfSemanticLayer.firstSequenceElement(elements, 120));
+
+        assertEquals(0, GsfSemanticLayer.firstSequenceElement(List.of(), -1));
+        assertEquals(0, GsfSemanticLayer.firstSequenceElement(List.of(), 120));
+    }
+
+}