WPF WebBrowser control: mouse wheel scrolling and Document.execCommand stops to work properly after I added event handlers

Question

I have a WPF WebBrowser control. After I added several events handlers in this way:

m_eventHelper = (HTMLDocumentEvents2_Event)Document;

m_eventHelper.onclick += new HTMLDocumentEvents2_onclickEventHandler(OnMouseClick);
m_eventHelper.onmousedown += new HTMLDocumentEvents2_onmousedownEventHandler(OnMouseLButtonDown);

mouse wheel scrolling doesn't work anymore (while mouse wheel event fired succesfully), as well as Document.execCommand, i.e.

Document.execCommand("SelectAll", false, null);