Fix subtle bugs.

Author: TomasMikula

undofx/src/main/java/org/fxmisc/undo/impl/UndoManagerImpl.java

@@ -207,13 +207,15 @@ private void addChange(C change) {
             if(merged.isPresent()) {
                 if (isIdentity.test(merged.get())) {
                     canMerge = false;
+                    queue.push(); // clears the future
                 } else {
                     canMerge = true;
                     queue.push(merged.get());
                 }
             } else {
                 canMerge = true;
-                queue.push(prev, change);
+                queue.next();
+                queue.push(change);
             }
         } else {
             queue.push(change);

undofx/src/test/java/org/fxmisc/undo/impl/UndoManagerTest.java

@@ -6,7 +6,9 @@
 import java.util.concurrent.CountDownLatch;
 
 import javafx.beans.property.SimpleIntegerProperty;
+
 import org.fxmisc.undo.UndoManager;
+import org.fxmisc.undo.UndoManager.UndoPosition;
 import org.fxmisc.undo.UndoManagerFactory;
 import org.junit.Test;
 import org.reactfx.EventSource;
@@ -63,6 +65,40 @@ public void testMark() {
         assertFalse(um.atMarkedPositionProperty().get());
     }
 
+    @Test
+    public void testPositionValidAfterAddingAChange() {
+        EventSource<Integer> changes = new EventSource<>();
+        UndoManager um = UndoManagerFactory.unlimitedHistoryUndoManager(changes, c -> c, changes::push);
+
+        changes.push(1);
+        UndoPosition pos = um.getCurrentPosition();
+        changes.push(1);
+        assertTrue(pos.isValid());
+    }
+
+    @Test
+    public void testPositionInvalidAfterMerge() {
+        EventSource<Integer> changes = new EventSource<>();
+        UndoManager um = UndoManagerFactory.unlimitedHistoryUndoManager(
+                changes, c -> -c, changes::push, (c1, c2) -> Optional.of(c1 + c2));
+
+        changes.push(1);
+        UndoPosition pos = um.getCurrentPosition();
+        changes.push(1);
+        assertFalse(pos.isValid());
+    }
+
+    @Test
+    public void testRedoUnavailableAfterAnnihilation() {
+        EventSource<Integer> changes = new EventSource<>();
+        UndoManager um = UndoManagerFactory.unlimitedHistoryUndoManager(
+                changes, c -> -c, changes::push, (c1, c2) -> Optional.of(c1 + c2), c -> c == 0);
+
+        changes.push(1);
+        changes.push(-1);
+        assertFalse(um.isRedoAvailable());
+    }
+
     @Test
     public void zeroHistoryUndoManagerMark() {
         EventSource<Integer> changes = new EventSource<>();