What prevents a figure showing before notebook cell completion?

My issue boils down to:

"Why does the figure not show immediately, but instead waits until the cell has completed running?"
```python
%matplotlib widget
import matplotlib.pyplot as plt
from time import sleep
fig = plt.figure()
plt.plot([1,2,3])
sleep(2)
```

This behaviour is preventing fun uses, like dynamic plotting in a single cell:
```python
%matplotlib widget
from matplotlib import pyplot as plt
from time import sleep
import numpy as np
fig = plt.figure()

for i in range(10):
x, y = np.random.random(2)
plt.scatter(x, y)
fig.canvas.draw()
sleep(0.1)
```

The previous code works if one splits it into two cells, placing the `figure()` call in the first one:

```python
# Cell 1
%matplotlib widget
from matplotlib import pyplot as plt
from time import sleep
import numpy as np
fig = plt.figure()

# Cell 2
for i in range(10):
x, y = np.random.random(2)
plt.scatter(x, y)
fig.canvas.draw()
sleep(0.1)
```

Edit: I also note that in the qt backend, there figure and plot are shown immediately, in contrast to the ipympl behaviour.

The issue names no source files or tests. Start by reproducing the single-cell and split-cell examples with the `%matplotlib widget` backend, then compare the `fig.canvas.draw()` behavior with the Qt backend; done means the cause and an appropriate rendering behavior are established.

Written by the indexing model from the issue text.