Enclosed is a standalone program demonstrating how GridPane breaks BorderPane somehow. The GridPane is always displayed at the leftmost part of the display.

I only tested this using JFXPanel -- it may not exist in a pure JavaFX program.

The enclosed images are: rectangles.png -- how it should look. And GridPanelMisbehave.png -- how it looks when I wrap the center rectangle with a GridPane.

(By GridPanel I mean GridPane -- sorry. The code is using GridPane exclusively).