As per spec of getPreferredSize() and getMinimumSize():
https://docs.oracle.com/en/java/javase/16/docs/api/java.desktop/javax/swing/plaf/basic/BasicSplitPaneDivider.html#getPreferredSize()
https://docs.oracle.com/en/java/javase/16/docs/api/java.desktop/javax/swing/plaf/basic/BasicSplitPaneDivider.html#getMinimumSize()

"Returns dividerSize x dividerSize"
It is not perfectly clear that dividerSize x dividerSize returns (expected to return) an instance of Dimension with height = dividerSize and width = dividerSize.

Also JDK implementation of getPreferredSize() and getMinimumSize() methods do not return new Dimension(getDividerSize(), getDividerSize()), rather returns instance of Dimension depending upon HORIZONTAL_SPLIT and VERTICAL_SPLIT.
Implementation snap:
if (orientation == JSplitPane.HORIZONTAL_SPLIT) {
    return new Dimension(getDividerSize(), 1);
}
return new Dimension(1, getDividerSize());