The default scrollWidth for a scintilla document is 2000. If SCI_SETCARETLINEVISIBLE is enabled and the scintilla view is larger than the scrollWidth, the caret line is clipped to the scrollWidth instead of using the whole width of the view.
There are other aspects of drawing like eolfilled backgrounds that also stop at the scroll width.
Unsure this should be classified as a bug. Does changing this mean that the document has logically unlimited width? Or does it just extend some drawing outside the document. Will there have to be similar changes at the bottom of the document, such as extending the final background colour?
I didn't see this behavior in Notepad++ for Windows so I just assumed this was a bug in the Cocoa version of Scintilla.
My only other experience with this feature is TextWrangler for the Mac which fills the background from edge to edge.
It may be worthwhile experimenting with this to see if it can easily be made to work as you want. I'm just a little cautious.
In your application, you might want to set scroll width to the current screen width when larger than 2000 as that should avoid the white area except when responsive scrolling goes beyond the end then bounces back. If this works well then it could become a default set by ScintillaView.