Correct the default plainChanges implementation and explain the quirk…

…y code.

richtextfx/src/main/java/org/fxmisc/richtext/EditableStyledDocument.java

@@ -20,10 +20,39 @@ public interface EditableStyledDocument<PS, S> extends StyledDocument<PS, S> {
     ReadOnlyStyledDocument<PS, S> snapshot();
 
     default EventStream<PlainTextChange> plainChanges() {
+        /*
+          The following code might seem a bit strange, but here's why it's used:
+            If a rich change is emitted that only changes the style, then inserted and removed are equal.
+            So, we want to exclude those style changes and only include changes where the actual text was modified
+            (e.g. some text was inserted/removed)
+
+            If we use the following code, "getText() is repeated twice:
+                richChanges().filterMap(
+                    // excludes style changes
+                    c -> c.inserted.getText().equals(c.removed.getText()),                          // first time
+                    c -> new PlainTextChange(c.position, c.removed.getText(), c.inserted.getText()  // second time
+                )
+
+            To prevent code repetition, we could map the RichTextChange to a PlainTextChange and then filter out
+              the PlainTextChanges that don't actually change anything (the RichTextChange was merely a style change)
+              However, this produces unneeded PlainTextChange objects.
+
+         So, to reduce code repetition and use less memory, the following approach is used.
+         */
         return richChanges()
-                // only allow changes that added/removed text; exclude style changes
-                .filter(c -> c.insertedLength() != 0 || c.removedLength() != 0)
-                .map(c -> new PlainTextChange(c.position, c.removed.getText(), c.inserted.getText()));
+                .map(c -> {
+                    String inserted = c.inserted.getText();
+                    String removed = c.removed.getText();
+                    if (!inserted.equals(removed)) {
+                        // if reached, the rich change was an actual text change, not a style change...
+                        return new PlainTextChange(c.position, removed, inserted);
+                    } else {
+                        // if reached, the rich change was a style change; text wasn't modified
+                        return null;
+                    }
+                })
+                // now filter out the null values (the style changes) to insure no null events are emitted
+                .filter(c -> c != null);
     }
 
     EventStream<RichTextChange<PS, S>> richChanges();