Feature request: visual indication of slide overflow

Related: #167 #159

If I add text to the top of a slide, sometimes it'll cause the content to overflow off the bottom of the slide. If I am making lots of quick changes all over the document, sometimes I don't notice the slide is missing text.

Some kind of visual indicator or error (how it'd be shown I don't know) when not all the slide content (particularly text) doesn't fit on the slide would be incredibly useful. If I know about it I can manually work around it by adjusting text size or splitting the slides.

If a line of text half fits on the bottom of a slide it's often noticeable, but if the bottom of the slide cleanly cuts off a line of text it's not immediately obvious.

I'm not a CSS person so not sure how difficult this sort of thing would be to implement. Hopefully not as difficult a problem as #159.