This is a follow up to JDK-8272229, which missed to mention that the reference implementation of ::oneTouchExpandableChanged behaves the following way on a variety of OpenJDK versions, starting from old ones:

        if (!DefaultLookup.getBoolean(splitPane, splitPaneUI,
                           "SplitPane.supportsOneTouchButtons", true)) {
            // Look and feel doesn't want to support one touch buttons, bail.
            return;
        }
        if (splitPane.isOneTouchExpandable() &&
            leftButton == null &&
            rightButton == null) {
            /* Create the left button and add an action listener to
               expand/collapse it. */
...


As a result, for example, with -Dswing.defaultlaf=com.sun.java.swing.plaf.gtk.GTKLookAndFeel installed, the buttons are not initialized.
Please see code sample attached.

When running the code on Linux:

> /jdk-21/bin/java -Dswing.defaultlaf=com.sun.java.swing.plaf.gtk.GTKLookAndFeel Test.java
divider.leftButton() = null
divider.rightButton() = null
splitPane.isOneTouchExpandable() = true
divider.leftButton() = null
divider.rightButton() = null
> /jdk-21/bin/java /scratch/jdk-cache/21/35/jdk-21/bin/java Test.java
divider.leftButton() = null
divider.rightButton() = null
splitPane.isOneTouchExpandable() = true
divider.leftButton() = javax.swing.plaf.basic.BasicSplitPaneDivider$1[,0,0,0x0,invalid,alignmentX=0.0,alignmentY=0.5,border=,flags=4194600,maximumSize=,minimumSize=java.awt.Dimension[width=6,height=6],preferredSize=,defaultIcon=,disabledIcon=,disabledSelectedIcon=,margin=javax.swing.plaf.InsetsUIResource[top=2,left=14,bottom=2,right=14],paintBorder=false,paintFocus=false,pressedIcon=,rolloverEnabled=true,rolloverIcon=,rolloverSelectedIcon=,selectedIcon=,text=,defaultCapable=true]
divider.rightButton() = javax.swing.plaf.basic.BasicSplitPaneDivider$1[,0,0,0x0,invalid,alignmentX=0.0,alignmentY=0.5,border=,flags=4194600,maximumSize=,minimumSize=java.awt.Dimension[width=6,height=6],preferredSize=,defaultIcon=,disabledIcon=,disabledSelectedIcon=,margin=javax.swing.plaf.InsetsUIResource[top=2,left=14,bottom=2,right=14],paintBorder=false,paintFocus=false,pressedIcon=,rolloverEnabled=true,rolloverIcon=,rolloverSelectedIcon=,selectedIcon=,text=,defaultCapable=true]