I know this have been requested numerous times the last couple years.
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viewtopic.php?f=4&t=6540&p=22778
viewtopic.php?f=1&t=5805&p=22360
And always rejected/postponed due to it's being hard to implement.
But how about "faking" it?
Right now there's the Editor Window (EW). A "fake" split would work like this:
In split mode the main window consists of the original EW, a splitter and a View Window (VW)
The EW is active and works as normal. The VW is just a snapshot (image) of the editor view at specific position.
Then when the user clicks the VW and makes that active, the two windows "swap place". So now the EW is in the position that was occupied by the VW and vice versa.
That way you only have to update the VW in certain cases.