Little crack when sliding on Iphone

Hi there. I found one little but annoying bug when using Safari on iPhone 13 pro to browse my website

When sliding or scrolling the thread page (not the home page), especially thread with images, there are always chances there is crack between the header and the notch of iPhone. It looks like this:

Here is one example link:

To figure it out, I use safe mode and disable all plugins and themes but the problem still exists.

Is there anyway to fix it? Such as using css to add a line, a background or something like that. Thank you very much!