Resizing extends outside the bounds of the notebook

Describe the issue

If you drag the resize handle outside the limits of the notebook and release the mouse you lose the ability to resize as the handle will be hidden:

This is a consequence of using a window mouse move listener: https://github.com/matplotlib/ipympl/blob/6b44a6f90cf74b12cfcf930b440010984d8a9b4d/js/src/mpl_widget.js#L272

Maybe there could be a check that it won't be resized to be larger the layout (this.model.get('layout'))?

Versions

 3.7.6 (default, Jan  8 2020, 19:59:22) 
[GCC 7.3.0]
ipympl version: 0.5.6
jupyter core     : 4.6.1
jupyter-notebook : 6.0.3
qtconsole        : 4.6.0
ipython          : 7.12.0
ipykernel        : 5.1.4
jupyter client   : 5.3.4
jupyter lab      : 2.1.5
nbconvert        : 5.6.1
ipywidgets       : 7.5.1
nbformat         : 5.0.4
traitlets        : 4.3.3
Known nbextensions:
  config dir: /home/ian/anaconda3/etc/jupyter/nbconfig
    notebook section
      jupyter-matplotlib/extension  enabled 
      - Validating: OK
      nbdime/index  enabled 
      - Validating: OK
      voila/extension  enabled 
      - Validating: OK
      jupyter-js-widgets/extension  enabled 
      - Validating: OK
JupyterLab v2.1.5
Known labextensions:
   app dir: /home/ian/anaconda3/share/jupyter/lab
        @axlair/jupyterlab_vim v0.12.2  enabled  OK
        @jupyter-widgets/jupyterlab-manager v2.0.0  enabled  OK
        @jupyter-widgets/jupyterlab-sidecar v0.5.0  enabled  OK
        @jupyterlab/git v0.20.0-rc.0  enabled  OK
        jupyter-matplotlib v0.7.2  enabled  OK
        jupyterlab_vim-system-clipboard-support v0.1.0  enabled  OK
        nbdime-jupyterlab v2.0.0  enabled  OK

阅读 js/src/mpl_widget.js 第 272 行附近的代码，并通过拖动超出 notebook 边界来复现 resize-handle 的行为。检查 window mousemove listener 如何使用 this.model.get('layout'); 当在 notebook 外部释放时不会让 handle 保持隐藏，也不会阻止继续调整大小时，该 issue 就完成了。