Fixed RichTextChange reported by ReadOnlyStyledDocument replace (#908)

Author: Jugen

richtextfx/src/integrationTest/java/org/fxmisc/richtext/api/UndoManagerTests.java

@@ -162,6 +162,17 @@ public void identity_change_works() {
                assertEquals(0, plainEmissions.get());
             });
         }
+        
+        @Test
+        public void testForBug904() {
+        	String firstLine = "some text\n";
+        	write( firstLine );
+        	interact( () -> area.setStyle( 5, 9, "-fx-font-weight: bold;" ) );
+        	write( "new line" );
+        	area.getUndoManager().preventMerge();
+            area.append( area.getContent().subSequence( firstLine.length()-1, area.getLength() ) );
+            interact( area::undo ); // should not throw Unexpected change received exception 
+        }
 
     }
 

richtextfx/src/main/java/org/fxmisc/richtext/model/ReadOnlyStyledDocument.java

@@ -419,7 +419,8 @@ private Tuple3<ReadOnlyStyledDocument<PS, SEG, S>, RichTextChange<PS, SEG, S>, M
             return start.map(l0::split).map((l, removed) -> {
                 ReadOnlyStyledDocument<PS, SEG, S> replacement = f.apply(removed);
                 ReadOnlyStyledDocument<PS, SEG, S> doc = l.concatR(replacement).concat(r);
-                RichTextChange<PS, SEG, S> change = new RichTextChange<>(pos, removed, replacement);
+                // Next we use doc.subSequence instead of replacement because Paragraph.concat's returned paragraph style can vary.
+                RichTextChange<PS, SEG, S> change = new RichTextChange<>(pos, removed, doc.subSequence(pos, pos+replacement.length()));
                 List<Paragraph<PS, SEG, S>> addedPars = doc.getParagraphs().subList(start.major, start.major + replacement.getParagraphCount());
                 MaterializedListModification<Paragraph<PS, SEG, S>> parChange =
                         MaterializedListModification.create(start.major, removedPars, addedPars);