Wrong scroll direction on Linux with OSR enabled.

[ionull]

Describe the bug
Scroll direction is reversed on Linux when OSR is enabled.

To Reproduce
Steps to reproduce the behavior:

1. Open a web page with JCEF (make sure it's long enough with vertical scroll bar)
2. Scroll with mouse wheel
3. Scroll down
4. It scroll up instead of scroll down

Expected behavior
Scroll as system side.

Versions (please complete the following information):

• OS: Ubuntu 22.04
• Java Version: java zulu-javafx-17.40.19
• JCEF Version: gc6a20bc
• CEF Version: 116.0.19

[magreenblatt]

Scroll direction is usually an OS configuration option. I'm not sure how that is read/represented in Java.

[1fxe]

I have a proposal for a somewhat hacky solution

This is the code in X11Window class responsible for making the mouse event.

if (xev.get_type() == XConstants.ButtonPress) {
    MouseWheelEvent mwe = new MouseWheelEvent((Component)getEventSource(),MouseEvent.MOUSE_WHEEL, jWhen,
       modifiers,
       x, y,
       xbe.get_x_root(),
       xbe.get_y_root(),
       1,false,MouseWheelEvent.WHEEL_UNIT_SCROLL,
       3,button==4 ?  -1 : 1);
    postEventToEventQueue(mwe);
}

So when scrolling down normally the button passed is 5 and delta is positive, X11Window does keep track of lastButton so in the native code we could fetch that and just reverse this behavior on linux

Made a POC that works, its just working in the case for natural scrolling, we would have to do the opposite of getWheelRotation() to calculate the delta properly

jclass xWindow = env->FindClass("sun/awt/X11/XWindow");
jfieldID lastButtonId = env->GetStaticFieldID(xWindow, "lastButton", "J");
jlong button = env->GetStaticLongField(xWindow, lastButtonId);
 
// Would check against getWheelRotation 
if (button == 5L) { 
   delta = -3;
} else {
   delta = 3;
}