compress (X11) mouse wheel events

There is one event per wheel step, so wheeling more can create a number
of wheel events, and LO processed those one by one. If processing one took
long (e.g. the repaint was complicated), the scrolling visibly lagged.

This commit works only for X11 VCL backend (and by extension, KDE3/4 backends).

Change-Id: I5eff7446da16167ec75925e75243314c68bc81a4

vcl/unx/generic/window/salframe.cxx

@@ -2718,6 +2718,51 @@ void X11SalFrame::SimulateKeyPress( sal_uInt16 nKeyCode )
     vcl_sal::getSalDisplay(GetGenericData())->SimulateKeyPress(nKeyCode);
 }
 
+namespace
+{
+struct CompressWheelEventsData
+{
+    XEvent* firstEvent;
+    bool ignore;
+    int count; // number of compressed events
+};
+
+static Bool compressWheelEvents( Display*, XEvent* event, XPointer p )
+{
+    CompressWheelEventsData* data = reinterpret_cast< CompressWheelEventsData* >( p );
+    if( data->ignore )
+        return False; // we're already after the events to compress
+    if( event->type == ButtonPress || event->type == ButtonRelease )
+    {
+        const unsigned int mask = Button1Mask << ( event->xbutton.button - Button1 );
+        if( event->xbutton.button == data->firstEvent->xbutton.button
+            && event->xbutton.window == data->firstEvent->xbutton.window
+            && event->xbutton.x == data->firstEvent->xbutton.x
+            && event->xbutton.y == data->firstEvent->xbutton.y
+            && ( event->xbutton.state | mask ) == ( data->firstEvent->xbutton.state | mask ))
+        {
+            // Count if it's another press (i.e. wheel start event).
+            if( event->type == ButtonPress )
+                ++data->count;
+            return True; // And remove the event from the queue.
+        }
+    }
+    // Non-matching event, skip certain events that cannot possibly affect input processing,
+    // but otherwise ignore all further events.
+    switch( event->type )
+    {
+        case Expose:
+        case NoExpose:
+            break;
+        default:
+            data->ignore = true;
+            break;
+    }
+    return False;
+}
+
+} // namespace
+
 long X11SalFrame::HandleMouseEvent( XEvent *pEvent )
 {
     SalMouseEvent       aMouseEvt = {0, 0, 0, 0, 0};
@@ -2936,13 +2981,23 @@ long X11SalFrame::HandleMouseEvent( XEvent *pEvent )
                     nLines = SAL_WHEELMOUSE_EVENT_PAGESCROLL;
             }
 
+            // Compress consecutive wheel events (way too fine scrolling may cause lags if one scrolling steps takes long).
+            CompressWheelEventsData data;
+            data.firstEvent = pEvent;
+            data.count = 1;
+            XEvent dummy;
+            do
+            {
+                data.ignore = false;
+            } while( XCheckIfEvent( pEvent->xany.display, &dummy, compressWheelEvents, reinterpret_cast< XPointer >( &data )));
+
             SalWheelMouseEvent  aWheelEvt;
             aWheelEvt.mnTime        = pEvent->xbutton.time;
             aWheelEvt.mnX           = pEvent->xbutton.x;
             aWheelEvt.mnY           = pEvent->xbutton.y;
-            aWheelEvt.mnDelta       = bIncrement ? 120 : -120;
+            aWheelEvt.mnDelta       = ( bIncrement ? 120 : -120 ) * data.count;
             aWheelEvt.mnNotchDelta  = bIncrement ? 1 : -1;
-            aWheelEvt.mnScrollLines = nLines;
+            aWheelEvt.mnScrollLines = nLines * data.count;
             aWheelEvt.mnCode        = sal_GetCode( pEvent->xbutton.state );
             aWheelEvt.mbHorz        = bHoriz;