Fix TaggedLineString to return correct vertex

modules/core/src/main/java/org/locationtech/jts/simplify/TaggedLineString.java

@@ -60,7 +60,21 @@ public int size() {
     return parentLine.getNumPoints();
   }
 
+  /**
+   * Returns a vertex of the component,
+   * in either simplified or original form.
+   * Once the component is simplified a vertex of the simplified linework
+   * must be returned. 
+   * Otherwise the simplified linework could be jumped by a flattened line
+   * which does not cross an original vertex, and so is reported as valid.
+   * 
+   * @return a component vertex
+   */
   public Coordinate getComponentPoint() {
+    //-- simplified vertex
+    if (resultSegs.size() > 0) 
+      return resultSegs.get(0).p0;
+    //-- original vertex
     return getParentCoordinates()[1];
   }
 

modules/core/src/test/java/org/locationtech/jts/simplify/TopologyPreservingSimplifierTest.java

@@ -249,8 +249,7 @@ private void checkTPS(String wkt, double tolerance, String wktExpected) {
     Geometry geom = read(wkt);
     Geometry actual = TopologyPreservingSimplifier.simplify(geom, tolerance);
     Geometry expected = read(wktExpected);
-    //TODO: add this once the "skipping over rings" problem is fixed
-    //checkValid(actual);
+    checkValid(actual);
     checkEqual(expected, actual);
   }