clear_out() and then display()，jupyter will show plain text representation.

[cashewnut]

Description

When I drag a IntSlider, I want to clear the output(plot) first and redisplay the IntSlider, but the IntSlider can't be shown.

Reproduce

environments Pycharm 2011.1

1. run next code

   
   def handler2(change):
   clear_output(wait=True)
   display(tests)

tests = widgets.Dropdown( options=task_ids, description='test:', disabled=False, )

tests.observe(handler2, names='value') display(tests)

![image](https://user-images.githubusercontent.com/14501861/116351841-dd4d8200-a826-11eb-9a43-8fed15d97a07.png)
2. select another option such as 40:
![image](https://user-images.githubusercontent.com/14501861/116351932-02da8b80-a827-11eb-9257-999de91106cb.png)
3. but if I comment ```clear_output```
![image](https://user-images.githubusercontent.com/14501861/116351991-1b4aa600-a827-11eb-8eda-dd07a83bcbde.png)

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## Expected behavior

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## Context

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- ipywidgets version <!-- import ipywidgets; print(ipywidgets.__version__)-->
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- Browser and version:

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[JosephHodes]

clear_output no matter where you put it clears the output