Hey!
Going through a little annotation project myself just now and realized that it might be super useful to have an Undo button, especially when in Active View. Once one gets into a flow, it is easy to make a mistake and then the sentence disappears from active view, so correcting that change requires going into document view, finding the sentence, and applying the correct label – a tedious effort that could easily be avoided through an Undo function.
Cheers, keep up the great work!
Leiwn