SwingUtilities3.applyInsets [1] has the following formula:

        if (insets != null) {
            rect.x += insets.left;
            rect.y += insets.top;
            rect.width -= (insets.right + rect.x);
            rect.height -= (insets.bottom + rect.y);
        }

This formula produces correct results if and only if rect.x and rect.y are zero.

This is a pre-existing issue from BasicMenuItemUI [2].

The issue was found during code review for JDK-8348760 [3].


[1] https://github.com/openjdk/jdk/blob/19a76a45e9c8616414830c865801660bc6761e92/src/java.desktop/share/classes/com/sun/java/swing/SwingUtilities3.java#L152-L159
[2] https://github.com/openjdk/jdk/blob/11aadc9d98d364b91114c028c7e2eff8de2f2bf0/src/java.desktop/share/classes/javax/swing/plaf/basic/BasicMenuItemUI.java#L782-L789
[3] https://github.com/openjdk/jdk/pull/23324#discussion_r2168852213