Redmine

add following code in my theme fixing this issue

* {
    --fonts-main: "Noto", Arial, sans-serif;
}

This flickering is likely a phenomenon called FOUT (Flash of Unstyled Text).

* { --fonts-main: "Noto", Arial, sans-serif; } is not properly loading "Noto Sans," so Arial is being used instead. Indeed, there is no flickering when using Arial, but I personally prefer the Noto Sans font. It's a dilemma.

The Noto Sans font is cached, so FOUT (Flash of Unstyled Text) is less likely to occur with regular reloads and only happens during a hard reload or on the first access. Therefore, I don’t find the flickering caused by FOUT particularly bothersome, but this might vary depending on the person.

Thank you for explanation.