only use the previewwindow if requested by the user

[GustavoKatel]

Tentative fix for #524

closes #524

[stevearc]

This is the correct approach to fix the issue, but why did you remove the calls to update_preview_window? I'm pretty sure that will lead to out-of-date previews in some cases.

[GustavoKatel]

but why did you remove the calls to update_preview_window?

late night coding. my bad :) fixed it

[stevearc]

Thanks for the PR!