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

js/src/mpl_widget.js の272行目付近を読み、ノートブックの境界を越えてドラッグして resize-handle の動作を再現してください。window mousemove listener が this.model.get('layout'); をどのように使用しているかを確認してください。ノートブックの外で離したときに handle が非表示のままにならず、それ以上のリサイズも妨げられなければ issue は完了です。