Preview does not work when working with splitted editor views

[vbrdev]

e.g. when working with Side By Side editor views.

[kreuzberger]

If preview is open, the editor area is splitted (e.g. side-by-side). The preview is then shown empty, but it gets updated if the file is changed.
If you toggle between two editor windows, the preview is not updated to the correct preview.
Every update works only on content change.