-----------8<-----------
SplitPane pane = new SplitPane();
    
StackPane stack1 = new StackPane();
stack1.getChildren().add(new Button("b1"));
    
StackPane stack2 = new StackPane();
stack2.getChildren().add(new Button("b1"));
    
pane.getItems().addAll(stack1, stack2);
pane.setDividerPositions(0.5f);
    
Scene secene = new Scene(pane);
stage.setScene(secene);
-----------8<-----------

Executing this code the pane should be devided 1:1 and the divider should be located in the center but it looks like the left most control is always getting the whole size and other components are only given their minimum widths.