Moved from geany-plugins because split window is part of the Geany repository.
Lots of things don't work in split window. Because its a plugin, Geany code doesn't know about the second editing window, and isn't written to handle it, so any operations that are implemented in Geany won't work. Operations that are not implemented in Geany and which fall through to the underlying Scintilla widget will still work.
Really the split window should only be considered as a read-only window and all Scintilla editing operations also disabled, or the split moved from a plugin to Geany and work properly, but "somebody" has to do it.