fix estimated scroll bidirectional binding (#79)

src/main/java/org/fxmisc/flowless/StableBidirectionalVar.java

@@ -0,0 +1,95 @@
+package org.fxmisc.flowless;
+
+import java.util.function.Consumer;
+
+import org.reactfx.Subscription;
+import org.reactfx.value.ProxyVal;
+import org.reactfx.value.Val;
+import org.reactfx.value.Var;
+
+import javafx.application.Platform;
+import javafx.beans.property.Property;
+import javafx.beans.value.ChangeListener;
+import javafx.beans.value.ObservableValue;
+/**
+ *  This class overrides the <code>Var.bindBidirectional</code> method implementing a mechanism to prevent looping.
+ *  <br>By default <code>bindBidirectional</code> delegates to <code>Bindings.bindBidirectional</code>
+ *  which isn't always stable for the Val -> Var paradigm, sometimes producing a continuous feedback loop.<br>
+ */
+class StableBidirectionalVar<T> extends ProxyVal<T, T> implements Var<T>
+{
+    private final Consumer<T> setval;
+    private Subscription binding = null;
+    private ChangeListener<T> left, right;
+    private T last = null;
+
+    StableBidirectionalVar( Val<T> underlying, Consumer<T> setter )
+    {
+        super( underlying );
+        setval = setter;
+    }
+
+    @Override
+    public T getValue()
+    {
+        return getUnderlyingObservable().getValue();
+    }
+
+    @Override
+    protected Consumer<? super T> adaptObserver( Consumer<? super T> observer )
+    {
+        return observer; // no adaptation needed
+    }
+
+    @Override
+    public void bind( ObservableValue<? extends T> observable )
+    {
+        unbind();
+        binding = Val.observeChanges( observable, (ob,ov,nv) -> setValue( nv ) );
+        setValue( observable.getValue() );
+    }
+
+    @Override
+    public boolean isBound()
+    {
+        return binding != null;
+    }
+
+    @Override
+    public void unbind()
+    {
+        if( binding != null ) binding.unsubscribe();
+        binding = null;
+    }
+
+    @Override
+    public void setValue( T newVal )
+    {
+        setval.accept( newVal );
+    }
+
+    @Override
+    public void unbindBidirectional( Property<T> prop )
+    {
+        if ( right != null ) prop.removeListener( right );
+        if ( left != null ) removeListener( left );
+        left = null;  right = null;
+    }
+
+    @Override
+    public void bindBidirectional( Property<T> prop )
+    {
+        unbindBidirectional( prop );
+        prop.addListener( right = (ob,ov,nv) -> adjustOther( this, nv ) );
+        addListener( left = (ob,ov,nv) -> adjustOther( prop, nv ) );
+    }
+
+    private void adjustOther( Property<T> other, T nv )
+    {
+        if ( ! nv.equals( last ) )
+        {
+            Platform.runLater( () -> other.setValue( nv ) );
+            last = nv;
+        }
+    }
+}

src/main/java/org/fxmisc/flowless/VirtualFlow.java

@@ -180,7 +180,7 @@ private VirtualFlow(
                 layoutBoundsProperty(),
                 b -> new Rectangle(b.getWidth(), b.getHeight())));
 
-        lengthOffsetEstimate = sizeTracker.lengthOffsetEstimateProperty().asVar(this::setLengthOffset);
+        lengthOffsetEstimate = new StableBidirectionalVar<>( sizeTracker.lengthOffsetEstimateProperty(), this::setLengthOffset );
 
         // scroll content by mouse scroll
         this.addEventHandler(ScrollEvent.ANY, se -> {