If output is too big, don't show it

Often when using a notebook, a student will type in something that will lock up the browser because it produces too much output. Notebook should refuse to display it (or truncate it).

The issue does not name a file, test, or entry point. Start by tracing how notebook cell output is rendered in the browser and determine an agreed size limit or truncation behavior. Done means oversized output no longer locks up the browser and the behavior is covered by an appropriate test.

Written by the indexing model from the issue text.