That's typically memory and/or video driver related. Have you updated your video driver recently? On Linux, a reboot usually fixes the problem (especially if video drivers have been updated without a reboot). On Windows, I've seen it more rarely. But it's never been consistent (or caused any real issues). It's not new to Sigil 1.6.0 by any means. You've reported it yourself RE the 1.4.3 version (
https://www.mobileread.com/forums/sh...5&postcount=14). It's been around intermittently since the switch to QtWebEngine.