This bug randomly happens. When I change to font from Oxygen Sans to something else, it goes away, but comes back when I change the font back to Oxygen Sans. Not sure if it really has something to do with the font, though. I attached an image that shows how it looks. Reproducible: Always
Created attachment 96131 [details] Smudgy text blocks
Previous bug #355677 was closed as RESOLVED WORKSFORME. I had the same issue sporadically appear for months with gnome-shell on X11. There are several bug reports in Fedora's bugzilla for it: https://bugzilla.redhat.com/show_bug.cgi?id=742776 , especially attachment https://bugzilla.redhat.com/show_bug.cgi?id=742776#attach_550877 . This issue has gone for me, but I don't know why. Maybe an update in some font rendering library. You could try changing your hinting settings, that sometimes helped.
Also have a look at bug #356680, with upstream bugs https://bugreports.qt.io/browse/QTBUG-42386 and https://bugreports.qt.io/browse/QTBUG-36976
This also happens to me, but usually in the Menu as opposed to anywhere else. This is on the latest Nvidia drivers.
Created attachment 97925 [details] Screenshot (showing both monitors) with the blocky text
Still present in Plasma 5.8.4. The kickoff plasmoid shows the same rendering errors as in attachment 97925 [details], while the kicker plasmoid is fine.
https://bugreports.qt.io/browse/QTBUG-36976
*** Bug 441477 has been marked as a duplicate of this bug. ***