Sometimes (not always) while browsing Stack Exchange I notice that top bar tries to escape its position, and an annoying margin appears:
It is not really predictable. It could happen when I check achievements or notifications, it could happen while scrolling, also happened to me when I clicked to ask this question.
That said, no custom settings. Latest Safari on iOS 15.4. Browser zoom 100%.
I believe it’s a “sub-pixel rendering” issue caused by the super-high resolution screen (which it is on the most of flagship smartphones of recent years)