Vertical spacer outside of viewport are not included

[fcollonval]

This is a follow-up of #673 seen when reusing the diff viewer in jupyterlab-git

The spacer widgets added to the editors to align the code are not added on lines outside the viewport. Hence it breaks line alignment when scrolling long diff.

[HaudinFlorence]

@fcollonval I tried to reproduce the issue of not having the spacers added on lines outside the viewport (using the master branch) but I dont' think I can observe it as you can see it on the following gif:

Maybe I missed something.

[vidartf]

@fcollonval Is this still an issue? Would you mind including some reproduction steps if it is?