The editor.ActiveView.TextAreaViewportBounds property returns the bounds of the editable text area, meaning where the text actually is, excluding margins and scrollbars. The bounds are relative to the editor view itself.
To get those bounds relative to SyntaxEditor (if you allow split views, since in that case the view won't be the same bounds as SyntaxEditor), you'd have to translate it up like:
In your screenshow, I believe #1 is the text area viewport bounds height.
#2 would be the #1, subtracting the two scrollbar button heights and the splitter height (if there is no split). The buttons should be a height of SystemParameters.VerticalScrollBarButtonHeight.