go to:
https://bootswatch.com/cerulean/
scroll to form and select the select element.
The navbar will disappear as long as the iOS Safari input helper is open and reappears when closing.
The same happens to ALL fixed elements on the page, they disappear\get relative positioned.

I have an SPA app which modifies the page contents when selecting an option and all elements stay hidden after closing after choosing a select option. The nav ans sidebar are seem to loose their fixed postition and stay at relative positions. Furtermore the postition of the select input helper is misplaced after opening the select element a second time. Turning the device repositions the select helper to the correct position.

Apple bug report has been filed. #28407863
generic bootstrap site before select

generic bootstrap site after select

web brf normal view before selecting select element

web brf nav sidebar scroll to top and all other fixed elements disappeared after selecting select element

web brf after closing select

web brf misplaced select helper

web brf after screen rotation select helper realigns