I've used KDE Plasma 5 with Centos and Kali and for several years on different hardware and this has been a consistent problem. After the browser has been open and in use for a day or so, there is a square area within the center of the displayed browser page that starts flashing some previous web-page contents. The flashing is rapid at about 8 times a second. The browser becomes lethargic and is barely functional enough for me to logout of a site and restart the browser. I can move the tab to a new window but this fixes it only briefly. Since it happens with Chrome and Tor (Firefox) browsers I realize it is not browser specific, but probably a KDE or OS problem. The only cure is to restart the browser. Eventually it goes bad sooner and I have to logout-login to KDE to get more time again. Using Kali 5.8.0, 16Mb, Intel i5 CPU.