When you are programming sites for use on mobile devices, setting up view ports in CSS can be a tricky endeavor. The units have been controversial for a while, and that is because of the way web browsers on mobile devices implement them. Different browsers seem to take different view ports into consideration when setting up the site's layout and loading the page, while others seem to ignore other page elements. For example, a scroll bar might be included on one but ignored in another browser's calculations. It is the problem of the developer to make sure that all of the view port units are considered when the layout of a page is calculated by a mobile web browser.
One way that a programmer can go about doing this is to take a look at the vertical height allowance. Items are displayed in 1 percent increments of the vertical height. However, that guidance does not take into account different screen sizes or browser layout requirements. Once you get past the browser's interface, such as its top menu or tool bar, then the vertical height immediately changes. Once a person scrolls to the content, there is often a frustrating shift in the display, causing the user to scroll past the first block of content and miss part of what the site has to offer.
Another way that a coder can set up the view port units is to use a browser that sets the vertical height as a fixed point. Safari has an easy way to do this. The mobile version of Chrome implemented a way to do this in 2017. Android's Firefox browser has an issue, and the firm is working on a fix to get it sorted. The problem with using a fixed vertical height is that the bottom of the view is cropped.