FIX: inconsistency between actual notebook output and Output.outputs

[maartenbreddels]

Fixes #2350
I think this approach of updating the outputs trait will always make the notebook's real output and the outputs trait the same.

[jasongrout]

This looks right to me. Do you have a concrete example/list of steps showing the difference between status quo and this PR? You mentioned refreshing in the issue?

[maartenbreddels]

import ipywidgets as widgets
from IPython.display import clear_output

output2 = widgets.Output()
with output2:
    print("in output2")
    clear_output(wait=True)
display(output2)

This will show in output2, but the widget state does not have the content anymore. If you refresh the notebook, nothing will be shown. With this PR it still is in the widget state and will be shown after a refresh.

[jasongrout]

With this PR it still is in the widget state and will be shown after a refresh.

And, just checking, if you then display one more thing after the refresh, it replaces the output (i.e., the pending clear is still in effect?)

[maartenbreddels]

No, the pending clear is not in any state, outside of the run of a single cell AFAIKT. For instance, using display/clear_display(wait=True) (without an Output widget) will only work in a single cell. This is consistent with nbconvert now as well jupyter/nbconvert#969 But I have not looked at the sourcecode of the classical notebook, this is purely observational.