Occurs when a navigation operation is beginning, shortly after the BeforeNavigate event.
For the WEB-BROWSER2 control only, EVENT-DATA-1 is set to 1 when the size is greater than 2 gigabytes, and EVENT-DATA-2 is set to the size of the download (modulo 2 gigabytes). Also, after this message, you can query W2P-PROGRESS-MAX to get the full value.