A DesktopTextArea has a property of VerticalScrollPosition to show the first viewable line within the DesktopTextArea.
Is it possible to retrieve the VerticalScrollPosition of a WebTextarea?
You’d need to open a Feature Request. This is one of those events that could fire very often and, as a result, was not implemented for Web to prevent overloading the server. It could be added in such a way that throttles how often the event is sent to the server, but that’s going to produce significant lag time in your application.
Given what it sounds like you’re trying to do from your other post, I would not attempt to directly port this code. The better path forward would be to build a solution using the WebSDK.
As my other post mentions, I will tone down my Web expectations.