Fix thin lines between cells and right side empty blocks

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

@@ -96,7 +96,10 @@ public void setTargetPosition(TargetPosition targetPosition) {
      * and re-lays out the viewport
      */
     public void scrollCurrentPositionBy(double delta) {
-        targetPosition = currentPosition.scrollBy(delta);
+        // delta needs rounding otherwise thin lines appear between cells,
+        // notably when scrolling with a mouse or using a scroll bar and
+        // usually only visible when cells have dark backgrounds/borders.
+        targetPosition = currentPosition.scrollBy(Math.round(delta));
         requestLayout();
     }
 

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

@@ -324,15 +324,17 @@ private void setHPosition(double pos) {
                 pos,
                 content.getLayoutBounds().getWidth(),
                 content.totalWidthEstimateProperty().getValue());
-        content.estimatedScrollXProperty().setValue(offset);
+        content.estimatedScrollXProperty().setValue((double) Math.round(offset));
     }
 
     private void setVPosition(double pos) {
         double offset = scrollbarPositionToOffset(
                 pos,
                 content.getLayoutBounds().getHeight(),
                 content.totalHeightEstimateProperty().getValue());
-        content.estimatedScrollYProperty().setValue(offset);
+        // offset needs rounding otherwise thin lines appear between cells,
+        // usually only visible when cells have dark backgrounds/borders.
+        content.estimatedScrollYProperty().setValue((double) Math.round(offset));
     }
 
     private static void setupUnitIncrement(ScrollBar bar) {