Horizontal scrolling with mouse buttons

I have a Logitech wheel mouse which supports horizontal scrolling by  
pressing the scroll wheel to either the left or right. These actions  
generate mouse button events where the button is identified as either  
"6" or "7" respectively.

Dillo handles these horizontal scroll events properly, but afterward  
the scrollwheel no longer works for vertical scrolling (the  
scrollwheel up/down events are reported as buttons "4" and "5"  
respectively); instead the scrollwheel results in left/right scrolling.

I am running Dillo 2.2 on a Slackware system.

Regards.