I just found after a fresh installation of openhab 3 and adding all Things and Items to a basic setup, that in the main UI overview on the page of the devices, the detail information windows are blurred e.g.
I get something a little similar on Firefox on Ubuntu 18.04. The popups eventually becomes sharp though - almost like it sometimes takes a few seconds to complete rendering!
I don’ know why, but now everything is fine again. I did deactivate the animations of the extended widgets under help but also turned it on again.
maybe sometimes the very basic thing helps again, like a say it in German:" Ein boot tut gut!" or in the common English meaning:“Has you tried to turn it off and on again?!”
So for me it works now since the last two days.
The animation suggestion may be a good try for others anyhow.