Fix propagation of fragments across multiple slides/subslides.

IPython/nbconvert/preprocessors/revealhelp.py

@@ -48,6 +48,12 b' class RevealHelpPreprocessor(Preprocessor):'
                 nb.cells[index].metadata.frag_number = index
                 i = 1
                 while i < len(nb.cells) - index:
+                    # We need to break the loop when a new slide or subslide is
+                    # found to avoid the propagation of the data-fragment-index
+                    # across multiple slides/subslides
+                    if nb.cells[index + i].metadata.slideshow.slide_type in ['slide', 'subslide']:
+                        break
+                    else:
                     nb.cells[index + i].metadata.frag_helper = 'fragment_end'
                     nb.cells[index + i].metadata.frag_number = index
                     i += 1