in RoyalTS/X it's possible to reorder open Documents, you can drag the Documents around in Royal TS but they are not "dropable".
if you want them to be reordered you need to close the Documents and open them in the right order.
Is it possible to add support for reordering already open Documents?
you're right, the documents cannot be dropped right now. I will look into it.
There's a workaround though: you can select the document and use the Move Up/Move Down command on Edit or Home to re-order the documents afterwards.
I hope this helps.