There's a bug here: the incremented copy of `_nextUntitledIndexToUse` isn't in the args list of the progress() callback, so it never tries with any number > 1.  The result is that right-click > New fails in any folder where a file named "Untitled-1" already exists.

The version on master increments `_nextUntitledIndexToUse` directly here.  Not the prettiest code on the block, but going back to that approach should still work just fine.