In a query tab I have a very long query (36 kilobytes) however some brief testing is showing this occurs with much shorter strings. It's a generated query by my web platform. I was using EXPLAIN to ensure that the automatically generated query would work without, obviously, executing it.
I've attached an image with two arrows to visually demonstrate this bug. The horizontal yellow arrow points at the keyboard caret which is at the very end of the line, obviously pressing the right arrow key to navigate forward would bring this textarea to the next line. The vertical red arrow points to the bug where text inappropriately appears.
It's not some kind of death-knell of a bug and it might be easy to fix. I just thought it was odd and a bit confusing at first. I'm using the latest stable version of HeidiSQL which is 184.108.40.20695 (obviously checked for updates). Hopefully you can directly reproduce the issue; I've been dealing with a couple client-only-errors myself of late and it's pointlessly frustrating; this bug is not terrible though but we can't let the world know that you're not completely perfect. ;-)