TAB completion does not work in Jupyter Notebook while another cell is running

Normally, TAB completion works fine in jupyter notebook.

However, if there is another cell running (e.g. a long running sub-process) TAB doesn't do anything until that and any other queued for execution cells won't finish their course. It does function in other notebooks, but not in other cells of the same notebook that's "busy".

To reproduce, run this cell:

import time
time.sleep(30)

then start a new cell, and try to autocomplete (`while`):

whil

No completion happens.

This behavior slows down the development process, and in a way somewhat defeats the idea of separate cells. As one can type code, but can't do it efficiently.

Thank you.

jupyter-1.0.0, python-3.6.6 here.

Reproduce the issue in Jupyter Notebook by running `import time; time.sleep(30)` in one cell and trying TAB completion for `whil` in another cell. Trace how completion requests are handled while cells are running or queued, then verify that completion works before, during, and after the long-running cell without delaying queued execution.

Written by the indexing model from the issue text.