Thanks for your replies. I did several tests with different browsers and different zoom levels. Here are the results:
PC running Windows 10:
- Chrome - as shown on above screenshot. Changing zoom level changes infill pattern.
- IE11, Edge and Firefox render correctly at all zoom levels.
- Safari 5.1.7 even failed to log into Octoprint. But I think this browser reached end of development few years ago and is outdated.
- Opera behaved the same as Chrome.
MAC running Mojave:
- Both Safari and Chrome render correctly at all zoom levels.
Apart from this IE11 failed to load webcam stream from /webcam/?action=stream complaining that something may not be configured correctly, but this can be a topic of other conversation.
To summarize: looks like the issue I reported in this thread in only present on PC an only under Chrome and Opera browsers. I definitely do not consider this as Octoprint bug and not planning to report it.