Closing a figure from the notebook does not close the python figure

This is essentially a regression of https://github.com/matplotlib/matplotlib/issues/4841 which happened when the nbagg backend was converted to a widget.

To reproduce create a cell with a simple plot.

plt.plot(range(10))
plt.show()

re execute the cell and observe that the figure count goes up and memory consumption goes up too. If you continue doing this you will eventually have more than 20 open figures and matplotlib will print a warning even if you only have one figure displayed.

The same is true if you explicitly close the widget using the widget close cross on the left of the figure.

I have spent some time trying to figure out how to best fix this but don't really know how to best do this. This function
is meant to trigger a close message to the python side when the figure is removed from the DOM but no longer works.

The old no longer existing close button is also intended to send a close signal to the python layer but this is not hooked up to the widget close button.

https://github.com/matplotlib/matplotlib/issues/6414 has some more related issues.

从 js/src/mpl_widget.js 开始，重点查看 issue 中描述的 DOM 移除函数和 widget 关闭按钮处理。通过反复执行绘图单元格并显式关闭 widget 来重现问题，然后确认 figure 数量、内存使用量以及超过 20 个 figure 的警告不再继续增加。