Empty 'Open Document' drop downs in C++ editor windows

Any new editing windows created by 'Split' or 'Split Side by Side' do not populate the drop down combo box on the window toolbar with the list of open documents. The drop down is only ever populated with just the file which was opened in the split.

The first editor window opened on project load does contain, and keep, the list of open documents. If this editor window is closed, occasionally a new split will contain the entire list–exactly when this happens is nondeterministic.