Anyone else getting blank areas in the replies?

Just recently I've noticed blank areas in the responses while scrolling through the replies. There isn't a line running along the left side indicating a response, or a series if lines indicating 'responses to responses'- just blank space.
Scrolling on a bit further everything returns to it's normal format.

Did I hit a wrong preference button, changing the way things are displayed?
This appears to be both on my desktop and phone.
TIA