You should use SplitPane.setResizableWithParent
static method.
SplitPane root = new SplitPane();
final Pane paneFixed = new StackPane();
paneFixed.getChildren().add(new Text("fixed"));
SplitPane.setResizableWithParent(paneFixed, Boolean.FALSE);
Pane paneFree = new StackPane();
paneFree.getChildren().add(new Text("free"));
root.getItems().addAll(paneFree, paneFixed);
stage.setScene(new Scene(root, 300, 200));
stage.show();
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