There already is a similar bug filed against Vaadin v7 Grid with PopupView and Valo theme:
Grid in popupview is not working with Valo theme - https://github.com/vaadin/framework/issues/7735
I created a new bug for Vaadin 8.0.1 with PopupView and linked the old issue related to v7 and valo theme.
Vaadin 8 Grid in PopupView not rendered correctly - https://github.com/vaadin/framework/issues/8793
In our application the behaviour with Vaadin 7.7.7 was OK and became an issue with the switch to Vaadin 8 as reported.
Can still be reproduced in the latest 8.0.1 release.