Created attachment 106083 [details]
comparision of disabled highlighting and enabled highlighting

This happens in all applications that use katepart4 and katepart5 (kate, kwrite, kdevelop, kile, ...). When I open kate source code highlighting is initially switched off. Now when I type some text (may be a Java class) and then select this text, no stripes are visible in the selection. But when I switch to some highlighting (in this case Java, but doesn't matter) and after that select the text again, one can see vertical white stripes in the selection. The stripes occur randomly before and after words. Now, even if I switch off highlighting, the stripes aren't going to disappear. I have to restart the application to get rid off them. 
Besides Arch I could reproduce it in KDE Neon. I tried with modesetting driver and intel xorg driver (SNA, UXA, DRI2, DRI3) but the problem persists. On reddit one user could also reproduce it on a RadeonSI system (https://www.reddit.com/r/kde/comments/6gm2wo/kate_shows_ugly_stripes_in_selection_when_text_is/diuaes8/).