RFR: 8372756: Mouse additional buttons and horizontal scrolling are broken on XWayland GNOME >= 47 after JDK-8351907

[Alexander Zvegintsev]

On Wed, 3 Dec 2025 00:04:43 GMT, Alexander Zvegintsev <azvegint at openjdk.org> wrote:

> The #25265 change introduced an artificial restriction that is actually unnecessary and harmful:
> 
> https://github.com/openjdk/jdk/blob/f5e4cd7f0d12fd21399b192b32a5c9abfe8a3564/src/java.desktop/unix/classes/sun/awt/X11/XToolkit.java#L1526-L1529
> 
>> Trying to click any mouse additional button raises an exception like the following:
>>
>> WARN - sun.awt.X11.XToolkit - Exception on Toolkit thread
> java.lang.IllegalArgumentException: Nonexistent button 6
>         at java.desktop/java.awt.event.MouseEvent.<init>(MouseEvent.java:774)
>         at java.desktop/sun.awt.X11.XWindow.handleButtonPressRelease(XWindow.java:765)
>         at java.desktop/sun.awt.X11.XContentWindow.handleButtonPressRelease(XContentWindow.java:45)
>         at java.desktop/sun.awt.X11.XBaseWindow.dispatchEvent(XBaseWindow.java:1206)
>         at java.desktop/sun.awt.X11.XBaseWindow.dispatchToWindow(XBaseWindow.java:1178)
>         at java.desktop/sun.awt.X11.XToolkit.dispatchEvent(XToolkit.java:939)
>         at java.desktop/sun.awt.X11.XToolkit.run(XToolkit.java:1086)
>         at java.desktop/sun.awt.X11.XToolkit.run(XToolkit.java:968)
>         at java.base/java.lang.Thread.run(Thread.java:1583)
> 
> This changeset simply reverses the check, returning it to its original state.
> 
> The actual check for the supported mouse buttons for the robot is maintained here.
> 
> https://github.com/openjdk/jdk/blob/f5e4cd7f0d12fd21399b192b32a5c9abfe8a3564/src/java.desktop/share/classes/java/awt/Robot.java#L367-L371
> 
> https://github.com/openjdk/jdk/blob/f5e4cd7f0d12fd21399b192b32a5c9abfe8a3564/src/java.desktop/share/classes/java/awt/Robot.java#L208-L214

src/java.desktop/unix/classes/sun/awt/X11/XToolkit.java line 1530:

> 1528:                 //If we have 3 physical buttons and a wheel, we report 3 buttons.
> 1529:                 //If we have 1,2,3 physical buttons, we report it as is i.e. 1,2 or 3 respectively.
> 1530:                 if (numberOfButtons >= 5) {

Suggestion:

                if (numberOfButtons > 5) {

It actually should be `>5`, without it the `numberOfButtons == 5` in `else if` branch is always false.

I want to put the fix to 26, so I intentionally left it as it was before because it had been tested for years. Once we agree on that, I'll file an issue.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28621#discussion_r2583185136