Created attachment 102327 [details] Screenshot of Kate to show the blurry font. The font in the text editor is blurry, see attached pic. The font in Kate's menu is fine. OS is openSuse Leap 42.2.
Is it because of the font size? Did you try other fonts? Could you please copy & paste the text you see in Kate into the area where you currently have "The Quick Brown Fox jumps...." and make a screenshot again?
I was about to write a comment because I just managed to solve the problem but I believe there's still some kind of bug. The problem is, Kate (the editor, the menu is fine) is not honouring the anti-aliasing settings for the font as set in Plasma 5 settings. Instead, it's using the systems anti-aliasing settings, in my case: Yast -> System -> Fonts. By changing the settings there, I was able to solve the problem. If it's technically not possible to let Kate use the Plasma 5 settings for anti-aliasing (in the text editor, the menu's are fine), shouldn't be there an advise in Kate's font setting about how/where to change anti-aliasing?
Hmm, Kate doesn't do anything special for the settings, it relies on Qt to use the right ones. If the plasma 5 settings are not taken, this looks more like a distro issue. As there is no code path in KTextEditor beside just plain "let Qt render the stuff", could you report this issue to your distro?