Fix scrolling for outputs.
Fix scrolling for outputs.

Fixes #4434

We still have a problem with “new view of output” cutting off the bottom bit, since the toolbar takes up a bit of room at the top of the panel and the height 100% output is pushed down.
jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab