@Luos If you right click your desktop and go to Display Settings, do you use any scaling beyond (or below) 100%, or do you use a DPI higher than 72?
I have to sit back a bit from my 4k monitor so scaling is at 125%, but haven’t run into that jumpy textbox issue (but I have experienced it in the past on 1080p a couple years ago).
p.s. Where did you find that dark theme? If it’s a custom CSS stylesheet you should try removing it temporarily to see if the textbox styling override is causing the problem.